Network Rail

Ref: Control Tables PJW

IRSE EXAMINATION

STUDY NOTES

CONTROL TABLES

AND

PRINCIPLES

Peter Woodbridge.

TABLE OF CONTENTS

51INTRODUCTION

1.1Background51.2Aim of Notes51.3Context51.4Warning: White Lies51.5Endnotes61.6Nomenclature61.7Level of Completeness61.8Errors and Omissions61.9Signalling Principles72CONTROL TABLES82.1What?82.2Why?82.3Presentation Style and Standards92.4Control Table Notes ($ and #)102.5General Note on Control Table Presentation:112.5.1ANDs and ORs112.5.2Brackets123POINT LOCKING143.1Introduction143.2Reasons for the various categories of locking143.3Dead track locking153.4Foul track locking153.5Interlocking with Signals- how?173.5.1Reserved by setting of route173.5.2Locked by signal clearance173.5.3Locked by train en route173.5.4Sectional Release Route Locking / Maintained Locking.183.6Interlocking with Signals- which?203.7Facing Points in Overlaps213.7.1Time of Operation Locking213.7.2Overlap locking213.8Restoration Alarm, Self- Restoration / Auto-Normalisation223.9Interlocking with other Points233.10Technicians Control (Disconnections).253.11Signallers Control (Individual Points Switch).253.12Anti-preselection.253.13Caterham locking264POINT CALLING274.1Hard calling274.2Soft Calling of Points284.2.1Utilisation of Soft Call284.2.2Practicalities of Soft Call284.3Automatic calling285Swinging Overlaps295.1Introduction295.2Basic Swinging Overlap295.3Swinging Overlap Counter Conditions, Tracks305.4Swinging Overlap Counter Conditions, Extra Point325.5Swinging Overlap Counter Conditions, Opposing Overlaps335.6Swinging Overlaps- varieties345.7Swinging Overlaps- summary346PICTORIAL SUMMARY OF POINTS35Categories of Point Locking356.2What locking, when366.3Recipe: Producing Points Control Tables377ROUTE SETTING397.1Route Setting Process Overview397.2Route Determination397.2.1Push-Button Ring407.2.21st & 2nd Button lists407.3Route Availability Checks407.3.1Points Correct or Free To Go417.3.2Track Conditions437.3.3Other Routes From the Same Entrance Signal437.3.4Route Locking447.3.5Preset shunt locking467.3.6Huddersfield locking477.3.7Miscellaneous488OPPOSING ROUTE LOCKING508.1Introduction508.2Direct Opposing, Back to Back signals508.3Direct Opposing, Extent of Opposing Locking518.4Indirect Opposing559Aspect Level569.1Summary569.2Route569.3Points579.4Tracks579.5Movement Authority limited at the exit signal589.5.1Exit signal lamp proved589.5.2Exit signal TPWS effective589.5.3Exit signal AWS effective5810Colour the Up and the Down signals and dot the O/Ls5911ENDNOTES.60

1 INTRODUCTION

1.1 Background

These notes have been prepared primarily for staff of Lloyds Register Rail intending to sit the IRSE Examination, module 3 Principles. It is hoped that they will:

prove useful to facilitate private study of the subject prior to the workshop sessions to discuss issues and perform practical Control Table exercises,

be sufficiently comprehensive to form the basis of notes to be used as an aide memoir and revision aid.

1.2 Aim of Notes

The aim is to collect relevant information together that is useful to tackle both the Control Table and the written questions of module 3 and which may also be relevant to questions in other modules such as module 1. Hence, whereas certain topics covered are unlikely to arise in the Control Table questions, the material is included since some appreciation of the subject is relevant for being able to address other questions which may arise.

These notes are not intended to be a substitute for detailed familiarisation with the relevant Railway Group and Network Rail Company Standards that need to be followed for real work, but should obviate the need to study these purely for exam purposes.

1.3 Context

This document has been written assuming a generic route setting Multiple Aspect Signalling System. Various varieties have been in use for over 40 years and of course there are differences between them due both to differences in technology and the changing railway environment (larger percentage of trains running at the higher speeds, more use of fixed formation multiple unit trains, less shunting and freight, less reliance placed upon experience of drivers and signallers etc.).

The vast majority of the information is applicable to most MAS installations currently in service. The intention has been to make the document compliant with 2004 practice although obviously most actual sites are not fully in accordance with this. There has probably been more change in standards in the last 5 years than there was in the previous 20!

1.4 Warning: White Lies

Some of the statements made in this document are deliberately over-simplistic as to explain all the exceptions to general rules would overcomplicate the issue and risk obscuring the main point. In doing this I subscribe to: A little inaccuracy is worth a ton of explanation. I have attempted to give indication when I have been economical with the truth by means of endnotes. Be aware that there are exceptions to most rules, but for the 90% understanding, not worth bothering with!

1.5 Endnotes

I have made extensive use of numbered endnotes to point to the equivalent of cul-de-sacs in the text, which I felt important to include for completeness, but are a bit of a diversion from the main issue being discussed. You can choose to read or ignore as you wish, but at least it makes it clear that you are performing a GO-SUB and then RETURN. When viewed on screen, the endnote appears if you place the cursor on the cross reference for a while.

1.6 Nomenclature

The text refers to track circuits (often shortened to track) for simplicity, but this includes train detection section via axle counter. Fundamentally the locking imposed is identical; although there are a few nuances of detail when considering respective failure modes, these can be ignored for the current level of detail.

1.7 Level of Completeness

My intention is to have made this comprehensive for the subject area of points and route locking; no doubt Ill realise that its not complete as soon as I distribute. If it proves useful I would plan to extend to cover other locking.

1.8 Errors and Omissions

I am afraid that there are bound to be some given the basis on which this has been put together; hopefully not too many nor too serious. I see this as a living document, so please advise if you think something is erroneous or could have been better expressed.

1.9 Signalling Principles

GK/RTGK/RT

0039Semaphore & mechanical Signalling

0007Alterations to Permissible Speeds0040Cab Signalling

0009Numbering of signalling equipment0041Track Circuit Block

0011Train Detection0042Absolute Block

0025Signalling Control & Display Systems0044Signalling trains onto occupied lines

0026Signallers Route Lists0051Single Line Control

0027Resetting & Restoring to service of signalling equipment0054RETB

0029Train Activated Warning Systems

0030Signalling Lockout Systems0060Interlocking Principles

0031Lineside signals and Indicators0061Shunters Releases, Ground Frames

0032Aspect Sequences0062Control of Points

0033Lineside signs0064Overlaps, Flank protection, trapping

0034Signal Spacing0065Train Operated Points

0035Layout of Lineside Signals0091DRA

0036Transition to / from lineside signalling

0038Signing of Permissible Speeds

GE/RTGI/ RT

8034Maintaining Signal Visibility7033Lineside Signs

8035AWS

8037Signal positioning and visibility

8217Managing risk introducing axle counters

GK/GN

0525Guidance Note, GK/RT0025

0611Guidance Note, Train Detection

0802Glossary of Signalling Terms

RT/E /C/RT/E/G

11004Symbols for Plans & Sketches00028Robust Train Protection

RT/E /S

10133TPWS Interface Design Reqs

10134TPWS Track sub system Equipment

2 CONTROL TABLES

2.1 What?

Tabular presentation in a standardised format,

Specify the functionality required of the interlocking; in the hierarchy Control Tables sit between the Signalling Scheme Plan and the actual design (relay room wiring, site application data etc.),

Control Tables consider the interaction between the various interlocking entities, e.g. Points, Routes, Signals (with their associated AWS and TPWS), Level Crossings, TAWS, Slots and Releases etc. The logic is applied and, provided the result is successful, then the associated controlled output function is operated. The outputs generally operate trackside equipment, but may be to interface with neighbouring interlockings, adjacent signalboxes or to provide outputs at the signalbox itself.

Control Tables should consider every controlled function (i.e. interlocking output) and list:

what conditions must be satisfied prior to the function being allowed to be operated,

what locking that function needs to impose to prevent the operation of conflicting functions.

2.2 Why?

There is something unique about every signalling layout:

there are National Rules enshrined in the Signalling Principles; Control Tables instantiate these for every function on the layout, spelling out in precise detail what the implementation of the Principles actually means for that site,

the Principles are mandatory but there are often a range of alternative options or elements which are only stated as desirable or are subject to risk assessment. On any particular layout there may need to be compromises; slavish adherence to the precise wording of one might mean that it is impossible to comply with another because of some unalterable constraint. Hence sometimes need to trade-off to get the best overall solution; an instance where signal engineering becomes more of a black art rather than a science.

Allows at a glance understanding of how the layout can be operated:

what moves are possible simultaneously,

how the interlocking should operate, (used in the detailed design/ check process, then as a cross check at the time of independent testing of the achieved functionality, as a record to consult in the event of an alleged incident).

2.3 Presentation Style and Standards

Many very dissimilar presentation styles were used historically- every BR Region and Signalling Contractor seemed to have their own until mid 1980s (and of course most installations in use perpetuate these historic standards).Indeed much debate re how technology specific Control Tables should be.

Purist view: High level, to specify essential locking requirements

Pragmatist view: Should summarise all controls which are needed for whatever reason and reflect the manner in which the technology actually achieves.

RT/E/C/11600

Gives technical guidance re Control Table completion (but of variable usefulness!)

RT/E/C/11202 (often referred to just as 202) is the current standard for compiling Control Tables, replaced former standard CP9 (evolved from SR A sheet per item style).

Explains conventions, use of # notes and brackets,

Defines the $ notes,

Gives the various Control Table blanks likely to be needed.

2.4 Control Table Notes ($ and #)

$ notes are universal on all standard Control Tables nationally.

You dont need to know them (and if you use you must define) for the IRSE exam, though, since they are a useful shorthand and part of the jargon, it is possibly worth attempting to learn those which you keep coming across. For now just recognise that they represent things which keep cropping up in the context of Control Tables.

For reference the most important of them are listed below (note I have not always given the official wording, but have paraphrased where I think it makes meaning clearer):

$2 = Emergency Replaced Auto Signal,

$7 = applicable only when Temporary Approach Control imposed,

$10=checked only at the time the aspect first clears,

$11=only allows the one aspect clearance,

$15= TORR as per comprehensive Approach Locking release,

$16= Raynes Park control (protects false Approach Release),

$28= points correct or free to operate,

$29= points give normalisation alarm if left Reverse,

$30= points self restore,

$31= when points are locked,

$32= points are set only,

$33= points are set and locked but not detected,

$35=PTO- see continuation on next sheet!

$36= unless points are locked only by an overlap that can swing,

$37=points are set and detected but not locked,

$44= route step up facility (Warner to Main) is provided,

$45= AWS proved to be not suppressed,

$50= and TPWS active or not required since signal off,

$56= TPWS proving included in signal RGKE indication,

$57= TPWS proving in a flashing blue failure indication,

$60= or signal is ready to clear (just awaiting approach release),

$91= operation of treadle causes track circuit to show occupied

# notes are specific.

They should be explained on the table itself or at the front of the book (particularly if they are common to many Tables or even the entire Scheme). A note may apply to a one-off circumstance for a particular route, or may be used consistently because no-one has yet defined a sorely needed national $ reference for the concept!

2.5 General Note on Control Table Presentation:

1. The comma is used between entries; it isnt strictly needed but it is good practice. It is to be read as the logical operator AND. The combination is only true if all the considered functions are true and this summation is the most common combination used in railway signalling.

2. In circumstances when the requirement is only for a minimum of one of the functions to be true, we combine by use of the word or to be read as the logical operator OR.

It may be that the functions grouped together are of the same type (e.g. two tracks listed in approach release) which tend to be referred to as alternatives

Perhaps more often the functions are of different types (e.g. a track in the aspect associated with a point condition, or tracks in the Approach Locking lookback associated with a signal ARAFOAL). In this case the main function (i.e. that of the column heading in which appears) is said to be conditioned out by the other function.

As far as strict logic goes there is no distinction between these, but it is useful phraseology; I tend to think of the conditions as being the valid excuse being offered for not including the tracks as per the previous examples.

3. Brackets are used hold together something which must be read as one sub-statement which would otherwise fall apart.

4. There is also a pecking order amongst brackets in order to ensure that complicated expressions are correctly interpreted. Use:

( for the innermost brackets, then

[to hold groups of the inner level together, and then

{ for the highest level braces.

2.5.1 ANDs and ORs

ANDsimply a comma ,e.g. Points Set, Locked & Detected: 9001, 9002

ORwritten as lower case ore.g. Points Set, Locked & Detected:9003 or 9004

e.g. 9001R, 9002R or 9003N means (9001R AND 9002R) OR 9003N

i.e. Either 9003 Normal or both of 9001 and 9002 Reverse

AND is of higher status than OR; note that if reading English, the comma tends to SEPARATE phrases but in logic it JOINS.

2.5.2 Brackets

As in the example in 2.5.1, we have to use brackets if we wish to force a different interpretation from that dictated by the hierarchy of logical operators. If what we wanted was: 9000 Reverse definitely and either 9002 Reverse or 9003 Normal we would have to write: 9000R, (9002R or 9003N).

If an expression is more complex and therefore requires brackets within brackets, then we need to make sure it is obvious which open bracket is associated with each close bracket, so we use different types as shown below.

There is no logical reason to use the { in the above example, but it is often done to hold an entire expression together, particularly if it gets disjointed by being spread over several lines of text, or strung out to get various terms in different boxes within the table to suit the presentation of that style.

A point availability expression relating to setting a route with a swinging overlap could look like:

{[(9021N or 9023R), (9024N, 9040N)] or (9039N, 9040R)}.

From this it can be inferred that 9040 must be facing points in the overlap (since they are the only ones to appear as alternatively N and R in different parts of the expression).

For the Reverse lie of 9040 the only requirement is 9039 Normal (trailing points beyond the facers).

For the Normal lie of 9040 there are three other sets of points involved. In fact 9024 are trailing points beyond 9040N but there is then a further facing point 9023 for which

the Reverse lie imposes no further requirements but,

the Normal lie leads to trailing point 9021.

Details of swinging overlaps are included in sections 5 and ??? but the key issue here is the care needed with: or , { [ ( ) ] } when reading or writing expressions. Never just concentrate on the individual items; also remember to consider how they are combined to make the whole expression.

3 POINT LOCKING

3.1 Introduction

Points are at the heart of any interlocking and therefore when learning the intricacies of a new layout (e.g. starting to check or test an interlocking) it is best to comprehend their locking first. It also seems a sensible place to start when comprehending interlocking in generality for the first time.

The interlocking considers each point number as the entity for locking purposes; if a point actually consists of a number of separately driven ends, these will always be commanded to move together. A point will not be considered to have achieved the interlocking intention until detection is achieved at all ends.

3.2 Reasons for the various categories of locking

The following sections in turn consider each broad category of locking:

locking by direct track,

(3.3, 3.4)

locking by signal route,

(3.5.1, 3.5.2)

maintained route locking,

(3.5.3, 3.5.4, 3.6)

time of operation locking,

(3.7.1)

defined overlap locking,

(3.7.2, 5.6)

swinging overlap / counter-conditional locking,(5.3)

self-restoration / auto normalisation,

(3.8)

point-to-point locking,

(3.9)

technicians control,

(3.10)

IPS locking,

(3.11)

anti-preselection

(3.12)

Caterham locking.

(3.13)

Hopefully the reason for preventing point movement whilst a train is currently passing over them is immediately obvious. Similarly it is easy to understand why the points should be locked prior to clearing a signal that gives a driver authority to proceed on a route over them. These are the two categories of fundamental locking which would certainly be expected in even the most basic interlocking system.

Other locking has generally evolved over many years of signalling experience in order to address various secondary risks and the reasons for them may not be quite so obvious. Indeed there are still mechanical signalling installations which dont enforce them by equipment at all, but rely solely on the signaller operating in accordance with instructions. Whilst the details of methodology of implementation (and even the extent of implementation) does vary between sites, all the above categories would be expected on any MAS installation, if of course the layouts track design and signal positions actually requires them.

3.3 Dead track locking

A point is always locked (i.e. prevented from moving in response to a request from signaller via IPS or route setting by signaller or ARS) when any track circuit section containing the points is not clear. Normally each end of points has one track section which completely covers it, but probably does not extend too much beyond it.

Where a point has several ends, it is usual but not always essential for there to be a separate TC for each end and hence a point often has two dead locking tracks and could occasionally have more. We try to take the dead locking tracks to a theoretical perimeter fence sufficiently far away to protect the points once an intrusion has been detected; this is not always possible and thats when we resort to foul track or Time of Operation locking to make up for the deficiency.

Dead track locking will always lock a point both-ways i.e. prevent point going from Normal to Reverse and also from Reverse to Normal.

3.4 Foul track locking

Figure 1 Foul TC901 points:

AB is the dead locking track

AC might be a foul track needing to lock 901 from Normal to Reverse, so we need to check. It depends on the position of IBJ 1 wrt the Clearance Point. This sketch has been drawn with the IBJ effectively opposite 902; so the guess would be that its tight but just clear, so AC is not a foul TC. If it had been drawn closer to the convergence at 901, the implication would be that AC is foul.

DB is also possibly a foul track, this time needing to lock 901 from Reverse to Normal. Assuming the running lines are at the usual nominal 6ft spacing, IBJ2 would certainly not be clear. Thus DB is a foul TC when 902R, therefore said to be conditionally foul.

AA simply cant be foul, its end-on, being at the toe end of the points.

902 points:

DB is the dead locking track

DC is obviously so far from the place where the diverging lines run parallel that it certainly is not a foul track needing to lock 902.

AB might be a foul track needing to lock 902 from Reverse to Normal, it depends on IBJ 2 position wrt the Clearance Point. Usually if an IBJ has to be squeezed in like this then it will be foul of both lines, but there may just be space to make it clear of either 901RtoN or 902RtoN but not both.

Control Table entries would look like:

PointTracks Clear

901Locks R to NAB, (DB or 902N)

Locks N to RAB, AC#1

902Locks R to NDB, (AB or 901N)

Locks N to RDB

#1 only if IBJ1 is not sufficiently far away from 901 that able to prove clearance.

Foul tracks always lock the points, one way only. Do note that the locking:

allows the points to swing to line up with the obstruction,

prevents the points swinging away from it.

Note that if point identity 901 had actually incorporated 902 as well so that the point ends always operate together as a crossover (901A and 901B), then AB and DB would both be dead tracks locking the point number. This scenario makes the actual position of IBJ 2 irrelevant. Although such IBJs almost invariable are foul, they arent considered such, since the foulness will never be exposed being safely contained within the crossover. Beware complacency however; always check that anything which appears to be a crossover is actually numbered accordingly before ignoring the foul nature of any intervening IBJ.

PointTracks Clear

901Locks R to NAB, DB

Locks N to RAB, DB, AC#1

#1 only if IBJ1 is not sufficiently far away from 901 that able to prove clearance.

3.5 Interlocking with Signals- how?

3.5.1 Reserved by setting of route

Points become locked as soon as a route which requires them is first set. This can be thought of as an initial reservation which holds the points in the desired position for as long as the signal entrance button light is illuminated. It is much like the basic mechanical locking between the levers of a mechanical signalboxs lever frame.

3.5.2 Locked by signal clearance

At the instant the signal actually clears, this reservation locking is reinforced by the imposition of Approach Locking. Hereafter the points continue to be locked even if the signalman cancels the entrance button. Thus although the movement authority has been withdrawn from the train, the route for that train is held because there is no guarantee that the train will be able to stop at the signal (which was off but has now been restored to danger).

3.5.3 Locked by train en route

In addition points need to be locked even after the train has passed the protecting signal until it has safely traversed them. Hence the locking directly imposed by the route itself is not sufficient as, in general, the dead track locking of the points does not extend up to the protecting signal on each possible approach.

It would be possible to make all track circuits from the point back to each of the protecting signals directly lock the points, i.e. include them all as dead-locking tracks.

There are two disadvantages however:

there is a greater risk of the points becoming locked due to track circuit failure,

if a track circuit traversed by trains moving towards the points is also used by opposite direction trains moving away, the locking of the point inevitably continues too long which unnecessarily restricts the efficient use of the junction layout.

Hence the concept of a track circuit not immediately itself directly imposing locking, but it only becoming effective after route used. This is known as sectional release route locking and gives the required directionality; indeed a track circuit failure of a non dead-locking track without a route set will not lock the points.

3.5.4 Sectional Release Route Locking / Maintained Locking.

Route locking actually incorporates all the different elements of locking discussed in 3.5.1, 3.5.2 & 3.5.3. Therefore, apart from the dead (and any foul) track locking, it is generally the only mechanism by which points are locked. This is a consequence of how the engineering has been contrived to work but it is important to understand the principles within the separate constituent elements; interlockings dont need to implement via route locking but do need somehow implement the three functions. Think of route locking as purely being the intermediary that most forms of interlockings use in order that signals (via their routes) lock points, each other and indeed other items such as level crossings, swing bridges and releases for Ground Frames or PLOD sections.

Principles of operation are:

one route locking element per track circuit section for Up moves and another one for Down moves,

setting of a route from a signal, de-energises the first USR, and the rest of them follow in a cascade like a stack of dominos (as each was being held up by the one earlier in the chain back to the first),

if the route is cancelled, the whole chain picks up again, unless the signal is approach locked (either never cleared or can safely release) otherwise it all remains locked until the A/L times out,

if however a track circuit isnt clear when the chain is able to repick, then the release of the locking cannot continue onto that section. Hence the locking is released only as far as the train has reached, but is maintained locked in advance of it. It is this behaviour which gives rise to the description sectional release and indeed the alternative nomenclature of maintained locking.

-

Figure 2 Route holding by maintained lockingThe previous diagram which shows the path through the layout taken by a train using route 12B:

Points 904 would be locked RtoN by the train up to and including track CC, and would thus have just become free,

Points 906 would be locked NtoR by the train up to and including track CB and thus would still be locked even though signal 12 could be ARAFOAL.

Points 907 would have the same locking as 906 since the requirement to maintain flank protection still exists until train safely beyond CB. [In reality, on this particular layout, the point end is highly likely to have been numbered as 906B anyway, but there are circumstances where such a point would be given a separate number].

The Control Table entries (not complete, but considering only signals 10 and 12) would be like this (on the assumption that each signal has one route to each of the lines disappearing off the right hand edge of the diagram):

PointsLockedTracks clearAfter route used

904RtoNBA, BB, CC12B, 12C

NtoRCD, CC10A, 10B

BA, BB12A

906RtoNCD, CC, CB, DC10B

BA, BB, CC, CB, DC12C

NtoRBA, BB, CC, CB12B

CD, CC, CB10A

Note

1. The route locking tracks are listed in running order from protecting signal to the points (i.e. NOT in alphabetical order).

2. If the tracks are exactly the same for more than one route, then there is a combined line of entry.

3. The sequence will always start at the entrance signal and end at the furthest track which always locks the points dead (in this context includes any foul track locking which, for the point call being considering, is deadlocking, regardless whether it also locks the points for the other call).

3.6 Interlocking with Signals- which?

Having understood what route locking is, we now need to know a little more about when it is to be applied. The following table attempts to describe the various relationships between a point and a particular route which needs to lock it; obviously any one point may be in various different categories as appropriate to the individual routes.

Figure 3 Route locking on pointsSituationDescriptionRoute locking presentationTime out

Facing or trailing point on approach to entrance signalNot locked by route in MAS practice

Facing or trailing point in route

i.e. 902 901Between entrance signal and exit signal on path that train is actually signalledList route, then all tracks in running sequence until last dead locking track,

viz:NO, locking effective until train has passed clear

901NtoR requires Route and Route locking Normal: 14A: BB

-

902NtoR requires Route and Route locking Normal: 14A: BB, BC

-

Trailing point in overlap

i.e. 904 if 903 wasnt present

Beyond the exit signal but within the overlap (EOL where applicable)[Note this route locking is sometimes referred to as overlap locking].

List route, then start with the tracks in the overlap up to but excluding the first dead locking track to be outside the brackets. Then go back to list all tracks from entrance to exit signal within bracketed condition, viz:YES, if train timed to stand at exit signal, then overlap is released, unless train has entered the overlap.

904NtoR requires Route and Route locking Normal 14A: BE (BB, BC, BD --------or BD occ for x secs )

Trailing point in overlap beyond facers

i.e. 904Above presentation ignores the presence of 903 points; obviously if these exist and are Reverse, 904 should not be locked. Swinging overlaps quickly start getting complicated, but in essence the locking on 904 would be as above but with 14A being replaced by (14A or 903R). See section 5.

Facing points in overlap

i.e. 903Significantly beyond the exit signal but within the overlap (EOL where applicable)For a simple swinging overlap, there is no locking required.

See section 5 for a short introduction to complex swinging overlaps.

Just beyond signalSee next section Time of Operation locking

Flank points

i.e. 900Protects route from SPADs elsewhere on layoutAs per the points in line of route to which this point is providing the flank. In the example above, once train has passed 902, 900 no longer needs to be locked

3.7 Facing Points in Overlaps

3.7.1 Time of Operation Locking

Facing points within an overlap are normally not locked and may be thrown at any time; this gives the flexibility to make other train movements. However if they are close to the signal there is a risk that they could be caught in mid-stroke just at the time when a train SPADs the exit signal and therefore encounters the points.

If the time taken for 903 points to throw would exceed the time taken for a train to reach the points once having locked them by the dead track BE, then Time of Operation locking" is imposed. This effectively locks the points once the signals berth track BD is occupied, thus giving the increased distance necessary to have enough time for the points to complete their movement having started. However the points cant be kept locked forever, so once train has timed to stand at signal 18, the locking is released again. In order to give directionality for those situations where the line is bi-directional, Time of Operation locking is implemented via route locking.

Figure 4 Time of Operation LockingNote however that it is not the same as usual route locking; with route 14 set and the train approaching them, the points remain free; they only become locked once BD is occupied. Also whereas route locking from a particular signal will either lock points NtoR or RtoN, by its very nature T of Op locking must lock points bothways. It is therefore recorded in a special section at the bottom of the 202 Control Tables:

When routes used:Track Clearor track occupied for time

14ABDBDt

The time to stand value t is calculated from the length of BD, just as for the release of overlap. To limit the period when the points are locked the berth track is generally relatively short, so typically t is in the 30 second region.

3.7.2 Overlap locking

If there are facing points beyond the exit signal there is at least the potential for more than one overlap position as there could be one defined for each lie of points. Often this is the case but sometimes not all possibilities are regarded as being valid, the most obvious example being where the facing points lead into sidings. If therefore a facing point has only one valid lie when there is an overlap, it is called and locked just as if it were a trailing point in that overlap and is sometimes called defined overlap locking.

More generally the facing points are allowed in either position and act as a hinge to select which overlap to use. On certain installations this is purely manual by the signaller operating the IPS, but it is now general practice to automate this and these are called swinging overlaps. These are a specialist subject in their own right, and since the calling and locking are intimately related, they will be treated separately in a section 5 to avoid getting bogged down here. For now just be aware that route locking is used to impose some extra controls necessary in various circumstances where swinging overlaps exist.

3.8 Restoration Alarm, Self- Restoration / Auto-Normalisation

Unlike in mechanical practice where points are always returned Normal once they are no longer required Reverse, in MAS practice points are just left where they are until next needed. However there are exceptions where it is particularly desirable to put them Normal as soon as possible, generally where they form trapping protection for sidings.

There are two approaches:

$29, Restoration Alarm: Giving the signaller an alarm if points are left Reverse (except when they are actually keyed Reverse on the IPS as this is considered a deliberate signaller act) after a signalled train movement has passed over them, so that they are reminded to restore to the safer lie,

$30: Self Restoration: Actually calling the points back to their safer lie and only giving an alarm if they fail to achieve detection.Neither of these are actually forms of locking of course, but both utilise route locking to implement them. There are risks associated with taking the signaller out of the loop (see section 3.12) and so the first approach is often adopted, except in the more extreme cases.

If we do adopt the $30, then we need to be very certain that the train really has passed over the points (by the nature of things, many of the points which we would want to self-restore are little used and rusty, and the quality of the P-Way may not be high). Hence we do not get the interlocking to call the points immediately but rather start a timer to ensure that the points remain free for a continuous period, rather than just suffering an instantaneous loss of train shunt at the critical time as the train is passing over the points.

In summary,:

self restored points:

feature automatic calling by the interlocking itself (rather than the lower SIL level control system) once points have been proved free of locking for a time but left reverse,

give a indication to the signaller when they are restoring and an audible alarm if they fail to do so,

are sometimes (but very often are not) depicted as have the self normalising facility, perhaps by a different coloured IPS key or background colour to the panel tile, so the signaller has a visual clue.

points with restoration alarm:

very similar logic to the above is involved but instead of actually calling the points all that happens is that the signaller is given a reminder to do so themselves.

3.9 Interlocking with other Points

Why would a point ever need to interlock with another point?

From one viewpoint they dont, and indeed true point-to-point is not a feature of many MAS installations. It can be viewed as an evolutionary hangover from mechanical signalling days, but it is currently experiencing a revival, at least in a modernised pseudo point-to-point form, so its worth understanding.

Example 1:

[Any movement over 938R would also need to traverse 937N- there is nowhere else to go!]

Figure 5 Point to Point, via the next point

Point-to-point would prevent 937 and 938 ever being Reverse simultaneously:

938NtoR would require 937N

937NtoR would require 938N

Such locking would not be applied to modern installations. However, much the same end result is achieved by making any route which needs to call 937R also to call 938N ; obviously any route which calls 938R would also be calling 937N anyway.

Example 2:

[Two diverging points within the same track- cant use both simultaneously Reverse]

Figure 6 Point to Point, two in a TCThe similarity is that there is no valid reason why both 939 and 940 would ever be required Reverse simultaneously. True point-to point would literally prevent the situation ever being possible, pseudo point-to-point effectively means that any route over one set Reverse is made also to call the other set Normal. The relevant aspects would generally only detect the points within line of route had actually responded to the call, but the other set would also be detected if they provide valuable flank protection within that particular layout.

Example 3.

[Scissors points]

Where much point work needs to be fitted into a confined area and the speeds are slow enough, scissors crossovers are sometimes provided- effectively a facing and trailing crossover overlaying each other. This is another case when there is no value on having one point reverse whilst the other is also reversed.

The routes over 941R are also made to call 942N, the routes over 941R are made to call 942N; some call this scissors locking. The other point number in this case gives no meaningful flank protection. I dont know the reason why we normalise the unused point number, but suspect it is because:

a)it has always been done,

b)it helps to make more routes lock out each other by virtue of requiring points in different positions as it saves having to perform the locking via opposing route locking and theres less risk of mistake in design and in degraded working.

Not that this is really a combination of examples 1 & 2- two point numbers are included within the same track circuit and also no valid reason for ever having both sets of points reverse simultaneously.

3.10 Technicians Control (Disconnections).

Not really regarded as point locking, but the technician must always have the facility to take a set of points out of use (i.e. be able to make a secure disconnection, having signed them Out Of Use with the signaller). This might be for Personnel Safety or alternatively it might be for System Safety.On all RRI installations the technician will always be able to find some link or fuse to remove to make the points safe, but on certain installations its made easier by providing a specific place to do this. On SSI and other software based systems, the disconnection is made from a keyboard and the disconnected status stored electronically. Hence on modern Control Tables, the entries for Technicians Control will always be YES.

3.11 Signallers Control (Individual Points Switch).

This is so obvious and basic that it doesnt feature on the Control Table, but it shouldnt be forgotten.

On a route setting panel, although the signaller does not often use them, there is an IPS for every point number allowing that entity to be controlled individually. Generally these switches are left in centre position which means the points are available for routes to call them as required; if the signaller specifically requires to lock the points then IPS would be turned appropriately. This prevents any routes which require points in the opposite lie, but not those for which the IPS position is appropriate.

Similar arrangement on VDU based control system but instead of IPS the points are always regarded available for route setting unless signaller has specifically required them in a certain position.

3.12 Anti-preselection.

This is something else which doesnt appears on any Control Table but is effectively a form of locking which shouldnt be forgotten.

It is a fundamental concept of an interlocking is that it forms a protection system between the signallers requests and the actual outputs (signal aspects, point positions etc) which only allows these requests to be acted upon if it is safe to do so. The corollary of this is: if a request is made at a time when it is not actionable, then it must not be stored and then acted upon later.

This would be called preselection; for example the signaller might have got ahead of themselves and asked the system to move points Reverse whilst the dead track was still occupied by a train. The request is of course represented by the position of the IPS which would remain as the signaller left it; anti-preselection is what stops the points moving when the track subsequently clears. The same applies for route setting; either the route comes at the time of route request, or it never will.

3.13 Caterham locking

Not only doesnt this appear on a Control Table, but it is not even locking at all in the true sense, but is included here for completeness.

The Normal lie of a crossover is almost invariably that which allows parallel moves to take place (assuming that both lies dont allow such moves as is the case in complex layouts). Hence on a two track bi-directional railway one would expect the Normal lie of a crossover to be as per the depiction of 919. Caterham locking is the exception which proves the rule and strictly it is applied (particularly on SR) at terminal stations at the end of a two track branch. The idea is that if a train at 99 decides to commit a SASSPAD and go off into the wide blue yonder at least itll be directed towards the running line signalled in that direction; obviously this requires not only the Normal lie of 920 to be opposite to the usual, but also for the points to auto-normalise.

Figure 7 Caterham Locking

The term Caterham locking has more recently come to mean considering the best lie of points in an overlap as part of the changes introduced by Robust Train Protection. See also the next section.

To current standards, more likely to make 920B a single ended set of points and make them self restore to the position to divert a run away to the right direction running line.

4 POINT CALLING

4.1 Hard calling

Points are called into the desired position by:

Individual Point Switch (not shown on Control Tables as obvious!)

Routes requiring them because:

in line of route,

giving flank protection to the route,

giving flank protection to the overlap,

need trapping protection from sidings,

conditioning out foul tracks to the overlap,

trailing points in overlap,

facing points in an overlap that can be used to direct an overrunning train the most appropriate way,

facing points in a complex swinging overlap,

facing points in another routes complex swinging overlap,

leaving points in a position could give problems later if they become locked in the wrong lie

As a general rule:

Hence the above list is only really summarising the various categories of locking that have already been discussed in sections 3.5 - 3.6.

The Points Control Table records what the routes do to the points.

The Route Control Table lists what points need to be checked to be correct or free to go as a precondition to the route itself being allowed to set.

It is therefore definitely a very good cross check to compare the point entries on the Route Tables against the route entries on the Points Tables. Similarly a cross check should always be performed that all routes calling a point one way are also shown as locking them against an opposite call. In addition it is also worthwhile cross checking the point detection proved in the aspect level against those checked in the route level for availability.

4.2 Soft Calling of Points

One of the exceptions to the above general rule is soft calling. Whereas hard calling is associated with locking (and is thus hard and fast), soft calling is not. It is used to move points to their best lie for a route when it is not absolutely necessary that they respond, but where risks are reduced if they are in a certain position.

4.2.1 Utilisation of Soft Call

Soft calling can be used to provide flank protection for a route which wouldnt normally be able to have it (as the ability to set another legitimate route whilst the first was in use would have been lost). We therefore settle for establishing the flank protection if we can and also accept that it may not be possible to keep it even if we do get it initially. The logic is that this has got to be better than not even trying to have it at all. It is particularly suitable for avoidance of convergence rather than head-on collisions.

Another role for soft calling is for setting points that actually lie beyond the limits of the overlap just in case a very significant overrun does occur following a SPAD. We cant lock up the railway to and from Penzance when we signal a train out of Paddington, but we can consider what would be best in the what if scenario. This has rarely been a practice in the past, but now we formally consider the situation and might decide to call certain key points to their best possible lie. It would obviously generally be far too restrictive to lock them, but just calling them to their best lie when first setting the route means that they will still be there unless a higher priority need has occurred in the interim, and arguably this achieves ALARP.

4.2.2 Practicalities of Soft Call

a # ref is used (since there is as yet no $ref defined) on the Points Table to denote that calling is soft rather than hard;

the route doesnt test the points for availability since we do want the route to set regardless,

therefore the availability check is undertaken at the time the call on the points is to be applied. [Note that anti-preselection applies; soft called points will not move if originally unavailable and then become free whilst the route remains set].

obviously there is no locking associated with soft calling.

4.3 Automatic calling

See section 3.8 re self restoration of points.

Points are not directly called by ARS; as its name implies, it is the routes which are set by the system and the interlocking of the point neither knows or cares whether it was signaller or ARS which set the route which calls and locks it.

5 Swinging Overlaps

5.1 Introduction

The IRSE Exam does not require detailed comprehension and competence in complex swinging overlaps (thankfully!). However you should be aware of the concept and have a broad understanding of the issues. In a sense this is getting ahead of ourselves, but it isnt possible to consider points and signal controls totally separately; we are talking about interlocking!

A swinging overlap is one in which there is at least one set of facing points which can act as the hinge to direct the overlap one way or the other.

Its a simple swinging overlap if there is no difference to the aspect controls of the signal in rear for the two lies of the facing point. There is nothing special about the point locking in this scenario.

If its not simple, then it is a complex swinging overlap; these actually range from the trivially complex to the horribly complex. The difficulties with resolving the point calling and locking escalate accordingly.

5.2 Basic Swinging Overlap

Example 1

Figure 8 Swinging Overlap, basic

The diagram above shows a trivially complex swinging overlap. Since there is an additional track circuit to be proved clear depending on the lie of 903, the aspect level for signal 14 has to reflect this:

Tracks clearPoints set & detected N or R

14BB, BC, BD, BE, (DZ or 903N)903

Theres no special point locking as all tracks potentially in the overlap are point dead locking tracks anyway but this is not usually the case so see next example.

There is no special point calling either and the overlap just swings due to the usual point calling of the various routes:

Forward Route Setting- various routes from 18 swing the overlap according their requirement

Routes over 903B end Normal- if there is an overlap beyond 18 over 903R, then it would just be moved out of the way automatically by the default calling of 903 since the B end is in line of route.

5.3 Swinging Overlap Counter Conditions, Tracks

Example 2

Figure 9 Swinging overlap, trackThe diagram above shows a slightly more complex swinging overlap. There is now a further additional track circuit DY to be proved clear when 903 are reverse but the main complication is that DY is not a deadlocking track for those points:

RouteTracks clearPoints set & detected N or R

14ABB, BC, BD, BE, (DY, DZ or 903N)903

PointTracks ClearSwinging O/L requires

TC clearMaintained lockingafter

903Locked R to NBE, DZ

Locked N to RBE, DZDY[BB, BC, BD or BD occ for t]-----14A

What this is saying is:

Once we have allowed 14 signal to clear (by allowing it the easement to prove that 903 are detected Normal as an alternative to requiring the tracks DY and DZ clear), we dont want to accidentally cause this to revert.

Hence we mustnt allow the signaller to move the points if it is only that point detection which is keeping 14 showing proceed. Therefore we must lock 903 if that extra track DY isnt clear. Obviously if there isnt a current requirement for an overlap theres no aspect reversion that needs to be guarded against and thus no need for this locking.

This is called counter-conditional locking, sometimes abbreviated to a counter. Having put the OR in the aspect level, we have to pay the price for it in the point locking.

Any one counter condition will obviously appear in either the NtoR or the RtoN for the points but there frequently is a different counter condition by the same route on the same points in its two directions. If in the diagram above BE extended only over 903A with a separate BF track up to the end of the overlap then there would be RtoN locking dependent on BF just like the NtoR dependent upon DY.

5.4 Swinging Overlap Counter Conditions, Extra Point

Example 3

Figure 10 Swinging Overlap, pointsThis is where it starts getting a bit more difficult because theres a set of trailing points beyond the facers. The issues are:

905 need to be called to go Normal and then subsequently locked NtoR whenever theres an overlap beyond 18 except if 903 are Normal,

903 need to be called to go Normal and then subsequently locked NtoR whenever theres an overlap beyond 18 whilst 905 are not Normal or free to go.

When a signaller attempts to set a route which reading over 905R, it not just the normal availability of these points which is relevant. If 905 are found to be locked Normal, the reason for this might be that there is an overlap beyond 18 but otherwise they could be free. This means a further evaluation is needed, to see if the overlap which is locking them can be swung away, which would then allow 905 to be moved. If this is indeed possible, a route which itself doesnt pass anywhere near 903 has to call them out of its way to free up the layout.

Its actually worse than this normally, since:

theres no guarantee that 903 are free to swing, since there is extra track BF that would impose counter conditional on 903RtoN if it were occupied,

potentially there could have been a further set of points in BF track as well,

there could even be a further overlap to swing prior to 903 being able to swing.

Hence what could seem to be a simple route over 905 could have a complex task to do involving several sets of points nowhere near it, just to work out whether it can get the points that it really needs and therefore whether it can itself set.

However in essence, as far as the point locking of 903 NtoR is concerned, there is a need to check the availability of 905 to go Normal and this is treated on a Control Table similarly to the test of the additional track in example 2.

Be aware though that when extra sets of points are involved, thats not the end to the story. Also remember that for every or somewhere, there is going to have to be a reciprocal counter condition elsewhere.

5.5 Swinging Overlap Counter Conditions, Opposing Overlaps

Example 4

Back to the original diagram again, but with a slight twist, we now show that the other line has a signal where the same points are in its overlap as well.

Figure 11 Swinging overlap, opposing overlap Whilst points are 903 are Normal there is no problem with having a route set up to 18 at the same time as one is set up to 19.

With no route set up to 19, there is no problem with 903 being in either lie and, if assume the tips of 903 are far enough from the BD/BE joint to avoid Time of Operation, no locking imposed by 14 on them.

Similarly with no route set up to 18, there is no problem with 903 being in either lie for an overlap beyond 19.

However there is a constraint. We must have:

either [No route to 18] or [No route to 19] or [903 Normal].

The dreaded ORs again. Where theres an OR, there needs to be a counter condition. Once the signaller has been allowed to set both routes, there is no degree of freedom left and therefore the third object in the equation mustnt be allowed to change state to where we dont want it. Control Table presentations vary depending on complexity, but the essence is shown below:

PointTracks ClearSwinging O/L requires

TC clearMaintained lockingafter

903Locked N to RBE, DZ-{[BB, BC, BD or BD occ for t]--------

---{[DW, DX, DY or DY occ for t]----14A}or ----21A}

Dont worry re the presentation; just be aware that with no extra tracks or points in either overlap, there is still a need for counter conditional because of the routes which are opposing to each other for one lie of the points.

Also be aware that:

14A route would have to call 903N if it was trying to set at a time when there was an overlap beyond 19.

Prior to this 14A would have had to ensure that 903 was Normal or Free or else it wouldnt itself have been allowed to set whilst there was an already established overlap beyond 19.

Obviously the converse applies for routes (and there could be several) up to 19.

5.6 Swinging Overlaps- varieties

Example 5

Figure 12 Swinging overlap, varietiesDepending on the layout and the risk assessments undertaken, the various possible overlaps beyond a signal may be regarded as:

a) Equally acceptable. When 14 signal is set itll take the overlap wherever 903 happen to be lieing if it can.

b) Preferred. When 14 signal is set itll try to get 903 points into the defined preferable position if they are available to move. The overlap over 903N is likely to have been defined as preferred as it takes up less of the layout than over 903R. Hence generally 14 would call 903 Normal but if they were locked Reverse at that time (perhaps the first train is just passing over them) it will be happy to leave them. Once the train has cleared the relevant tracks the aspect will then clear.

c) Only if FRS Certain of the overlaps may only be regarded as valid when the forward route from the signal is set in that direction. The traditional scenario where this applied was the overlap beyond a signal which could read into a goods line or siding. It is now becoming far more widespread and the intention is to try to ensure that any overrunning train is at least retained on, or directed onto, a line signalled in that direction. Hence the overlap shown ending at 905 points may possibly be put in this category if that line was predominantly used by trains in the other direction and almost certainly would be if the line was not signalled in that direction.

5.7 Swinging Overlaps- summary

In general swinging overlaps consist of examples 2,3, 4 & 5 all rolled in together and probably more than one of each case. If an area has one swinging overlap it is very likely to have a few. It is surprising how quickly the complexity mounts and even quite an innocuous layout can be a struggle. Throw in a few foul joints, split-end detection (or even worse, crossovers separately numbered as single enders) with conditionally opposing overlaps and it becomes a complete nightmare.

If you have one at all in the IRSE exam, it will either be simple or the most straightforward type of complex. I suggest youd still pass if you ignored it and could get a credit if you recognised it for what it was but didnt quite know how to design it. Know about them in outline but dont get bogged down in them.

Time to leave it there and move on.

6 PICTORIAL SUMMARY OF POINTS

6.1 Categories of Point Locking

6.2 What locking, when

Figure 13 Point locking, what and when6.3 Recipe: Producing Points Control Tables

Ingredients:

Signalling Plan

Supply of preferred type Control Table Blanks

Pencil and rubber

1. Take a quick look at the layout:

look out for any features (point-point, flank, T of Op, defined O/L etc) that give the flavour of the layout and are there to separate the sheep from the goats,

use the route boxes listing all routes from a signal to see what reads to where and for what classes of route,

make your own route destination list; on the berth TC of a signal write down all the routes which read up to it.

2. Only then start doing a Control Table. Write Point Number in Title Box and your Candidate Number on the sheet. Assuming CT based on 202 fill in by columns left to right.

3. Write down the deadlocking track(s); there are likely to be two for a crossover, one for a single end. These lock the point both ways so put in both the N>R (top) and R>N (bottom).

4. Look for foul track circuits at the two diverging directions at each point ends.

Ask yourself does dead locking TC extend beyond the Clearing Point? If not, the adjacent TC must also lock the points one way only (to prevent a train movement over the other lie).

Next think is this track always foul, or is there another set of points which would give parallel moves in one of its lies?. If so, add ( or points not in foul lie).

Then think but whats locking those other points to prevent them changing subsequently. Perhaps jot down in margin of other CT to act as aid memoir to come back to later so as not to get distracted now.

5. Look for point to point scenarios. Ask yourself Is there a non-useful combination of point lies between obvious pairs of points?: Two point numbers within same TC?

Scissors crossover?

Double slips?

Double Junction type arrangement with fixed diamond?

No proper entry on Control Table yet but write a note in its margin and/or on the Signalling Plan next to the points: All routes calling points X to N/R (delete inapplicable) also need to call points Y to N/R (delete inapplicable).

6. Now list the routes which call the points N>R and then R>N. Think of all the categories:

In Route,

In Overlap,

Flank to Route (and possibly Overlap),

Trapping from sidings or Goods to Route / Overlap,

Beyond Overlap (to be really clever and up to date!)

Pseudo point-point- i.e. that extra calling in step 5.

When you think youve found them all, make yourself perform a series of cross-checks by thinking:

Are there different route classes to same destination (M/W/C/S/P)?

Have I focussed on one end of a crossover and forgotten all about the other end? Did I consider a signal on each possible approach to each end of the points? Did I think to go a further signal section back on every such approach when the points are in overlap? Run a quick health check: briefly consider all other routes listed in route boxes for all signals that so far feature. Use as trigger to make sure no others missed by mistake Do the same for all the exit signals with the points on there approach by casting your eye down the list you made on their berth TC. Justify to yourself that all routes not already on the CT are correctly absent rather than just forgotten.7. Having found all the calling routes, move onto the next column that lists the locking routes. They will be the same (excepting any soft calling). All the routes calling the points one way must lock them to make sure that they cant be taken back the other way again; hence put the same routes in the diagonally opposite box.

8. Now think of what route locking tracks need to be associated with each of these entries:

Within the route: list tracks in sequence starting with 1st TC beyond the signal up to and including the last deadlocking track. Dont forget this will include both TCs for a crossover Reverse and could go beyond the actual point if the next TC is still foul. Within the overlap: leave a space, ( list tracks from 1st TC in route in sequence up to the berth of the exit signal then put the berth TC in the next column and choose a sensible time ). Then go back to space youve left and write in the overlap TCs but in this case dont write in any TC already in the deadlocking column.9. Finally ask yourself Are the points facing points within an overlap? If they are, think:

Are they very close (50m then wouldnt be provided, if d