Overrun Protection and Mitigation - RSSB Iss 5.pdf · implemented in Track Circuit Block areas.)...

Submitted by ................................................................................................. R Genner Nominated Responsible Manager Approved by ................................................................................................. Philip Wiltshire Chairman, Train Control & Communications Subject Committee ................................................................................................. Michael Harwood Chairman, Operations Standards Subject Committee Authorised by ................................................................................................. Dr. M Walter Controller, Safety Standards

Railway Group Standard GK/RT0078 Issue Five

Date February 1998

Overrun Protection and Mitigation

Synopsis This standard defines the safety critical requirements associated with the provision of Overrun Protection and Mitigation. This document is the property of Railtrack PLC. It shall not be reproduced in whole or in part without the written permission of the Controller, Safety Standards. Published by Safety & Standards Directorate Railtrack PLC Railtrack House Euston Square London NW1 2EE © Copyright 1998 Railtrack PLC

Withdrawn Document Uncontrolled When Printed

jtrousdale

Text Box

Signatures removed from electronic version

This page is intentionally blank



Railway Group Standard GK/RT0078 Issue Five Date February 1998 Page 1 of 16

R A I L T R A C K 1

Contents

Section Part A

Description Page Issue record 2 Responsibilities 2 Compliance 2 Health and Safety Responsibilities 3 Supply 3

Part B 1 Purpose 4

2 Scope 4 3 Definitions 4 4 Principle 5 4.1 Basic Overrun Protection 5 4.2 Enhanced Overrun Protection 5 5 Requirements for Basic Overrun Protection 6 5.1 Overlaps 6 5.2 Flank Point Setting 8 5.3 Flank Track Circuits 9 6 Requirements for Enhanced Overrun Protection 9 6.1 Assessment of risk 9 6.2 Selection of Mitigation Measures 10 6.3 Change Control 10 7 System Failure 10 APPENDIX A - Criteria for Use of Trap Points 11 APPENDIX B - Criteria for Use of Automatic Train

Protection 13 APPENDIX C - Criteria for Use of Trainstops 14 APPENDIX D - Reduced Length of Overlap 15 APPENDIX E - Risk Mitigation Measures 16 References 17




2 R A I L T R A C K

Part A

Issue record This standard will be updated when necessary by distribution of a complete

replacement. Amended or additional parts of revised pages will be marked by a vertical black line in the adjacent margin. Issue Date Comments 1 Dec. 1992 New Document 2 Sept. 1993 Alternative retrospective action added. 3 Nov. 1995 Emergency Revision. Supersedes GS/ST 0078, Issue 2. 4 Mar. 1996 RA Indicators included. 5 Feb. 1998 Includes Overrun Protection. Replaces the

following SSPs which are superseded: SSP20 SSP30

Responsibilities Railway Group Standards are mandatory on all members of the Railway Group* and apply to all relevant activities which fall within the scope of each individual’s Railway Safety Case. If any of those activities are performed by a contractor, the contractor’s obligation in respect of Railway Group Standards is determined by the terms of the contract between the respective parties. Where the contractor is himself a duty holder of a Railway Safety Case then Railway Group Standards apply directly to the activities described in his Railway Safety Case.

* The Railway Group comprises Railtrack and the duty holders of Railway Safety Cases accepted by Railtrack.

Compliance The provisions of this Railway Group Standard are mandatory for design and implementation work undertaken on schemes for which signalling scheme plans are approved on or after 6th June 1998. For schemes for which, on 6th June 1998, there is an approved signalling scheme plan but which are not yet commissioned, Railtrack shall undertake a risk assessment where required in section 4.2 and shall incorporate Enhanced Overrun Protection where this is shown to be necessary by the risk assessment. Such incorporation may be undertaken as an additional and later stage of the project where the risks of delaying planned stages are assessed as being greater than the residual risk of commissioning without it. Such Enhanced Overrun Protection shall be commissioned by 6th June 1999. No retrospective assessment is required in respect of Basic Overrun Protection. On existing infrastructure, there is no requirement for retrospective action in respect of Basic Overrun Protection. In respect of Enhanced Overrun Protection at ‘at risk’ signals (as defined in Section 4.2) existing on 6th June 1998, a risk assessment shall be undertaken (and consequential action taken, as necessary) when:-

• more than one SPAD occurs within a twelve month period;

• a major change of traffic pattern occurs which is qualitatively assessed as likely to increase the risk of SPAD;

• other factors become relevant which are qualitatively assessed as likely to increase the risk of SPAD.




R A I L T R A C K 3

Health and Safety Responsibilities

In authorising this standard, Railtrack PLC makes no warranties, express or implied, that compliance with all or any of Railway Group Standards is sufficient on its own to ensure safe systems of work or operation. Each user is reminded of its own responsibilities to ensure health and safety at work and its individual duties under health and safety legislation.

Supply Controlled and uncontrolled copies of this standard may be obtained from

the Catalogue Secretary, Safety & Standards Directorate, Floor 1 Railtrack PLC, Railtrack House, Euston Square, London NW1 2EE. Telephone: 085 78774 or 0171 557 8774 (BT) Facsimile: 085 79072 or 0171 557 9072 (BT)




4 R A I L T R A C K

Part B 1 Purpose To define the safety critical requirements to control the risk of collision between

an authorised train movement and a movement overrunning a signal at danger.

2 Scope This standard applies to all stop and shunting signals:

• on passenger lines,

• on non passenger running lines unless otherwise specified by Railtrack,

• at all interfaces between passenger and non passenger lines.

This standard applies to all forms of signalling systems and all means of overrun protection and mitigation. It does not include the requirements for proving of foul track circuits in point and signal controls, which are covered in GK/RT0060.

3 Definitions As Low As Reasonably Practicable (ALARP) - (as applied to this standard): the reduction of likelihood and consequences of a SPAD to a level below which the costs and/or constraints of further mitigation measures outweigh their benefit. Area of Conflict - the sections of line on which a collision may occur, for example:

• the section of line from the fouling point at a convergence to the destination signal.

• the short section at a diamond crossing between each fouling point,

• single or bi-directional line sections,

• interlaced tracks,

• restricted parallel moves due to insufficient clearance. Authorised Movement - A movement made with the authority of the signalling system or of the signaller. Basic Overrun Protection - Protection applied to signalled movements to avoid collision with unauthorised movements. It includes overlap protection and flank protection by point setting and track circuit proving. Double Block - (as applied to this standard) A method of working in Absolute Block territory where the forward section is required to be clear before a train can be accepted from the signal box in rear. (Double Block working may also be implemented in Track Circuit Block areas.) Enhanced Overrun Protection - Additional protection applied to selected signals to mitigate the risk due to SPADs by reducing both their likelihood, and their consequence. Flank - Points or track circuits, not in the line of route but included in its controls to remove or reduce the consequences of a SPAD. Layout Risk Model (LRM) - A means of assessing the risk due to a signal being passed at danger. Overlap - The distance ahead of a stop signal up to which the line must be clear, and where necessary locked, before the signal next in rear can clear: • Full Overlap - The overlap required to allow an unrestricted approach to a

signal at danger.




R A I L T R A C K 5

• Reduced Overlap - An overlap of reduced length, where the permitted approach speed is below a specified level, which still allows an unrestricted approach to a signal at danger (see Appendix D).

• Restricted Overlap - An overlap of reduced length, available only when the

preceding signal is subject to approach control (delayed yellow). Platform Starting Signal Risk Model - A means of assessing risk due to a platform starting signal being passed at danger. (This method is particularly applicable to the re-assessment of risk at platform starting signals e.g., due to a change in conditions). Release Speed - relates to the use of a warning class route and is the speed of a train approaching a warning class route entrance signal at danger when the signal clears to a caution aspect, such that the train speed through the route can generally be uniform (until braking for the exit signal is required) and the restricted overlap at the warning class route exit signal is sufficient to mitigate the risk of overrun of the exit signal through driver misjudgement. Secondary Hazard - A hazard created by the application of measures to prevent or mitigate an existing hazard. SPAD - Signal Passed At Danger. Track Circuit Interrupter - A rail mounted device normally positioned at catch or trap points which maintains a track circuit(s) in its occupied state after the passage of a vehicle which may have been derailed. Trap Points - Facing points provided at an exit from a siding or on a converging line to arrest an unauthorised movement.

4 Principle Overrun protection shall be provided in accordance with the requirements of this standard to mitigate the consequences of passing a signal at danger due to:

• misjudgement of braking on the approach to a signal at danger. Mitigation measures for this hazard shall be known as Basic Overrun protection

• disregard of the danger signal aspect. Mitigation measures for this hazard shall be known as Enhanced Overrun protection

Overrun protection provides insufficient mitigation for the disregard of warning aspects at the signal(s) preceding the signal at danger. Measures for mitigation of disregard of warning aspects in signals include AWS, ATP, TPWS etc., and train handling techniques. Overrun protection and mitigation shall not be provided on non passenger running lines only where a suitable and sufficient risk assessment has demonstrated that the level of risk is acceptable. 4.1 Basic Overrun Protection

Basic overrun protection of a signalled route shall be provided in the following form:

• an overlap, in accordance with section 5.1, for all protecting signals, and

• by the setting of flank points or the inclusion of flank track circuits in accordance with sections 5.2 and 5.3 respectively.

4.2 Enhanced Overrun Protection

A risk assessment in accordance with section 6 shall be undertaken to determine whether enhanced mitigation measures are required to reduce the risk of collision or derailment to As Low As Reasonably Practicable (ALARP). Such an assessment shall be necessary at:




6 R A I L T R A C K

• Platform starting signals protecting a conflicting route ahead

• Other signals from which trains regularly start from rest and which protect a conflicting route ahead

• Signals protecting junctions where there is extended opportunity or time window for conflict, e.g. entering single line sections

• Other signals presenting high risk, e.g. where a distracting feature (a station, level crossing, rising gradient or speed restriction) intervenes between a signal and a preceding caution signal, or where the signal has a history of SPADs.

The risk assessment shall be undertaken using the:

• Layout Risk Model; or

• Platform Starting Signal Risk Model (GK/RC0578); or

• any other system which is acceptable to Railtrack.

Formal risk assessments for enhanced overrun protection are not required in respect of:

• signals purely providing protection from following trains

• passing loops on single lines worked by any token or staff method (including RETB).

5 Requirements for Basic Overrun

Protection

5.1 Overlaps

5.1.1 Provision

In track circuit block areas an overlap shall be provided beyond every stop signal which has a main route reading up to it. Movements signalled under main route conditions shall only be permitted to approach the signal concerned if this overlap is unoccupied and other movements are prevented from entering it. Refer to GK/RT0060 for overlap locking requirements.

In other forms of block working, protection against overrun shall be provided beyond the home signal (clearing point) and beyond an intermediate block home signal. In addition, an overlap may be provided beyond any colour light stop signal which is at least minimum signal spacing distance beyond a colour light signal(s) capable of displaying a caution aspect. (Such provision can avoid approach control of the preceding signal.)

The same overlap space shall not be used for two opposing or conflicting routes at the same time.

It is not permitted for the overlap beyond a signal on a passenger line to lead into a goods line or siding unless either:

• a route is set from the signal into the goods line or siding, or

• the goods line or siding is proved clear for the required overlap distance and the trapping arrangement is proved to be effective.

A separate overlap track circuit or equivalent train detection device shall be provided to indicate to the interlocking and the signaller that the overlap has been entered. This requirement may be waived (e.g. in respect of automatic signals) where :

• no route locking is necessary, and

• there is not a permissive move up to the signal, and

• the signal is not within the strike-in point of an automatic level crossing, and

• the signaller is not likely to be misled by a combined berth and overlap indication.

Overlaps are not necessary:




R A I L T R A C K 7

• in other than track circuit block areas, for a stop signal beyond the home signal and within station limits, providing that the preceding signal is subject to approach control (enforced by the signalling system or by instruction)

• in respect of non block signals (e.g. signals provided solely for level crossing protection) - where specified by Railtrack. Where reasonably practicable, non-block signals should be located overlap distance clear of potential obstructions, e.g. level crossings.

5.1.2 Length

The length of the overlap beyond stop signals shall be based on an assessment of the likely extent of any overrun past the signal at danger due to misjudgement by the driver. The overlap length shall generally be as follows:

Caution Signal(s) Stop signal Overlap Length Remarks

Colour Light Colour Light 180m (200 yards)

Colour Light Semaphore 180m (200 yards) 400m (440 yards) in fog or falling snow

Semaphore Semaphore 400m (440 yards) Double block in fog or falling snow implemented by Signal Box Instruction

Where trainstops are in use and the distance required for any tripcock fitted train travelling at the permitted speed to come to a stand after being tripped by the trainstop is less than the overlap length quoted above, the shorter distance may be used. Arrangements must be in place to control the risks associated with any non-tripcock fitted train travelling on the line.

Where it is not practicable to provide the required length of overlap, the provision of trap points in conjunction with an acceptable arresting arrangement may be used as an alternative. Such applications shall be subject to risk assessment which must consider the secondary hazards of derailment at the trap points and within the arresting arrangement.

Any other reduction in overlap length shall be based on an assessment that the extent of overrun due to driver misjudgement can be contained within a shorter overlap. Each case shall be authorised by Railtrack, as Infrastructure Controller.

In track circuit block areas with colourlight signals, where the approach speeds are low, overlaps of the length shown in Appendix D will generally be considered sufficient to mitigate the risk of overrun.

Permissible speeds shall not be locally reduced to permit a shorter overlap to be provided.

Where trains are required to approach a signal at danger beyond which the available overlap is less than the required length, a warning class route with approach control may be provided from the signal in rear reading up to the signal with the restricted overlap.

The point at which the aspect of the signal in rear is released from red shall be calculated to ensure that the speed of the train approaching the signal in rear is reduced to a level at which, assuming that the train does not then accelerate between the signals, it will approach the signal with the restricted overlap at such a speed that the restricted overlap is sufficient to mitigate any likely misjudgement by the driver in bringing his train to a stand at the signal. The release speed shall be decided so as to minimise the likelihood of drivers accelerating sharply between the signals but shall not generally be more than 30 mph.

When an approach controlled signal is to be provided the risk assessment shall include consideration of the secondary hazard of the driver anticipating clearance of the approach controlled signal when it is being held at red, and so passing it at danger.

Where a warning class route with a restricted overlap is selected, the overlap shall automatically step up to the full length with immediate clearance of the outer signal, upon setting of the forward route, provided that:




8 R A I L T R A C K

• the signal has not yet cleared, and

• there is no train in the route, and

• the full overlap is clear.

If a signal with an individual automatic working facility which applies to a route with a delayed clearance and restricted overlap is set to work in automatic mode, then it shall revert to a warning class route with restricted overlap following the passage of a train for which the full overlap conditions applied.

5.1.3 Shunt Routes

The length of the overlap for routes up to position light or semaphore shunt signals leading to running lines shall be:

Shunt route to Conflicting move Overlap Length Remarks

Shunt signal { Passenger 180m (200 yards) May be reduced or { to 45m (50yd)#1 LOS { Non Passenger 45m (50 yards)

Running Signal#2 None Required #3

# 1 Where specified by Railtrack to afford operating flexibility and subject to risk assessment.

#2 Colour light or semaphore

#3 An appropriate overlap may be specified by Railtrack, e.g. where regular propelling moves or a mixture of through and stopping movements take place. 5.2 Flank Point Setting

Flank points, where available, may be set to divert an unauthorised movement away from an authorised route, subject to there being a net reduction in risk by so doing. The line onto which the unauthorised movement is diverted should preferably be either:

• used predominantly for trains in the same direction, or

• a siding.

However, the two ends of a crossover which are arranged to operate together are normally acceptable as flank protection.

The setting of flank points may be omitted when:

• other routes are set which would divert an unauthorised movement away from the area of conflict into the path of another train, or

• other permissible traffic movements would be restricted, or

• a net reduction in risk would not be achieved, or

• a signalled movement is in progress away from the area of conflict via the points in their non-protecting position, or

• the distance from the flank points to the conflict point of the authorised route is greater than 200m (220 yards).

The normal method to protect a passenger line from a non passenger running line shall be an overlap to passenger line standards. Where the provision of an overlap is not practicable, trap points in the non passenger line shall be provided.

Trap points shall also be provided where other suitable points do not exist for the protection of passenger lines from sidings.

Trap points may be provided in other situations after risk assessment, as part of the overrun mitigation process. Guidance on criteria for the use of trap points is given at Appendix A. 5.3 Flank Track Circuits

In the absence of suitable flank points, any track circuits located beyond the protecting signal(s) on a converging line may be proved clear in the controls of




R A I L T R A C K 9

routes requiring overrun protection.

It should be borne in mind that flank track circuits are less effective than flank points in providing overrun protection, since they detect the hazard to the authorised movement, rather than preventing it. Such provision may also introduce secondary hazards due to right side failure of the flank track circuits, resulting in the protected signal having to be passed at danger. However, the adverse effects of such a failure can be minimised by the overrun feature requiring sequential operation of flank track circuits (e.g. first track occupied after berth occupied) for it to be effective. Consideration of these factors shall determine the extent to which flank track circuit protection is applied.

Control of the protected signal by flank track circuits may be inhibited when a signalled movement is in progress away from the area of conflict.

6 Requirements for Enhanced Overrun

Protection

6.1 Assessment of Risk

Railtrack shall determine the signals to which enhanced overrun protection (in accordance with section 4.2 above) applies and shall assess the risk consequent upon each of them being passed at danger. These assessments shall be undertaken by competent persons who understand the operation of the layout and the appropriate risk assessment techniques.

Some or all of the following factors may need to be considered in respect of the approach to and within the area of conflict:

• permissible speeds

• frequency of movements

• possible seasonal variation in railhead condition

• the permanent way layout

• the location of signals and adequacy of sighting

• the type of the traffic at the present time and any foreseeable changes

• gradients (i.e. rising or falling) and other topographical features such as tunnels, embankments, stations and other structures

• the provision of vigilance equipment on trains using the route

• the time window during which a collision could occur

• the likelihood of unauthorised movements being brought to a stand before reaching the area of conflict

• the ability to stop an authorised movement before reaching the area of conflict

• the severity of a possible collision

• secondary hazards created by diversion or derailment of the unauthorised movement

• secondary hazards created if the overrun protection fails to operate correctly (e.g., during degraded modes of operation)

• history of previous SPADs and incidents

• the possibility of drivers becoming distracted at the critical time (e.g., due to the performance of platform duties)

• the incidence of a level crossing in the overlap. 6.2 Selection of Mitigation Measures

Railtrack shall consult with the Train Operators which work over the route to determine appropriate mitigation measures for each of the signals giving rise to high levels of risk.

Risk mitigation can be achieved by measures applied to the infrastructure and/or to traction units which address either the likelihood or the consequence, or both




10 R A I L T R A C K

the likelihood and the consequence of a collision or derailment.

A selection of practical risk mitigation measures is given in Appendix E. They may be used singly or in combination according to the circumstances at each particular site. Guidance on criteria for the use of two of the possible measures, automatic train protection (ATP) and trainstops, is given in Appendices B and C respectively

For new projects the Layout Risk Model (or equivalent method acceptable to Railtrack) shall be used to determine the change in risk due to the deployment of the proposed mitigation measure(s). For existing signalling installations, the Platform Starting Signal Risk Model (GK/RC0578) may continue to be used.

The benefit accruing from the deployment of any combination of risk mitigation measures is computed by the reduction of risk (in terms of equivalent fatalities) compared with the whole life cost of providing those measures.

When suitable measures have been identified and agreed, Railtrack and the Train Operators shall implement them in a prioritised and coordinated manner. 6.3 Change Control

Railtrack shall review and if necessary repeat the assessment in the event of any material change (e.g. significant timetable alteration) which may affect the acceptability of the risk so as to ensure that it remains ALARP.

Railtrack shall maintain auditable records for each signal identified as requiring enhanced overrun protection measures. These shall include:

• risk assessments undertaken

• overrun protection measures applied

• monitoring of their effectiveness.

7 System Failure The failure modes of equipment adopted for overrun protection and mitigation measures shall minimise the creation of secondary hazards and their effect on authorised movements. Secondary hazards and their effects shall be considered during the process detailed in section 6.

Railtrack shall ensure that controls and/or operating procedures are in place to safeguard authorised movements during the time that the overrun protection and mitigation measures are ineffective. The signal authorising a movement under these conditions may be permitted to clear without such protection.

For example, the cold proving of the lamp filaments in a SPAD indicator is considered to introduce secondary risks to the associated signal that are greater than the primary risk that the SPAD indicator bulb will fail/will have failed when the SPAD indicator is required to work. Hence cold proving of SPAD indicator lamp filaments is not incorporated into the aspect controls of the associated signal.




R A I L T R A C K 11

APPENDIX A - Criteria for Use of Trap Points

A.1 Mandatory Requirements

Trap points shall be provided to protect passenger trains from overruns from sidings. They shall be located so as to:

• guide derailed vehicles away from the passenger lines, structures and any other hazards

• keep derailed vehicles, as far as is practicable, upright and in line

Trap points may also be provided to protect passenger lines from non passenger lines where other measures, e.g. an acceptable overlap, are not practicable.

A derailer may be provided in lieu of trap points, where no running movements approach the protecting signal and speeds are low. A 2 Use as a SPAD Mitigation Measure

Trap points may be provided as part of SPAD mitigation management:

• on converging passenger lines, unless an acceptable overlap can be achieved by other means

• at the approach to moveable bridges, in the absence of other protection

• at the exit from terminal stations and bay lines at through stations, if gradients or other circumstances make such provision desirable

• at the exit from passing loops on single lines where the overlap beyond the protecting signal is inadequate and trains are to be permitted simultaneously to enter the loops

• at stations with a falling gradient steeper than 1 in 500, and where vehicles are left unattended.

Where trains are allowed an unrestricted approach to trap points where there is an inadequate overlap, an acceptable arresting arrangement, e.g. a sand drag, shall be provided. A.3 Restoration of Trap Points

Arrangements shall be made for trap points to be restored to the trapping position after completion of a movement through them. This shall normally be by operation of the signaller’s controlling device. A visual and/or audible indication may be provided.

Exceptionally, where there is a serious risk of the running line being fouled by unauthorised movement of vehicles, automatic restoration after a suitable time delay may be provided. A.4 Track Circuit Interrupters

Where trap points occur in track circuited lines, a track circuit interrupter shall be provided. When operated, it shall:

• cause the track circuit through the trap points to indicate “occupied” (“undefined” where the interlocking permits)

• cause appropriate track circuit(s) on line(s) which may be fouled by a derailment at the trap points to indicate “occupied” (“undefined” where the interlocking permits)

• place or maintain at danger the aspects of any signals reading over the affected track circuits, which are not replaced by the track circuits becoming occupied.

Track circuit interrupters may also be located at buffer stops to perform a similar function.

Protection shall be provided against intermittent disconnection of the track circuit interrupter wrongly allowing Train In Section Proving (TISP) or Train Operated Route Release (TORR) conditions to be satisfied.





APPENDIX B -

Criteria for Use of Automatic Train

Protection

B.1 Typical Requirements

An ATP system should typically be provided where:

• train speeds exceed 200 km/h, or

• Automatic Train Operation (ATO) is provided, or

• a Moving Block system is in operation, or

• the headway requirement is greater than 30 trains per hour and the permissible speed is greater than 50 km/h.

ATP shall also be considered when complete route modernisation of both infrastructure and rolling stock is being undertaken. B.2 Use as a SPAD Mitigation Measure

An ATP system may be provided on a line of route basis as a means of reducing to ALARP the risk from SPADs. B.3 Provision of Overlaps

Where automatic train protection is provided, overlaps of sufficient length shall be provided to enable the required safety performance of the train protection system to be satisfied, having regard to the required release speed approaching each signal or block section limit marker.





APPENDIX C -

Criteria for Use of Trainstops

C.1 Typical Requirements

The requirements for the layout, operation and control of signalling systems on lines employing trainstops are dealt with in GK/RT0017.

As an alternative to ATP or other SPAD mitigation measures, trainstops may be considered for use on a line of route basis on lines where trains running at the maximum allowable approach speed are capable of being safely stopped by operation of the trainstop within the calculated overlap without risk of significant injury to passengers or staff and:

• any collision would result in a high risk of loss of life or injury and serious damage to infrastructure, or

• signalling and stations are in confined spaces (such as underground railways in single bore tunnels) or are raised (such as overhead railways), or

• the service headway and incidence of conflicting moves frequently require trains to approach signals at danger.

Trainstops may also be considered for use on lines where the majority of overlaps can accommodate the stopping of a train passing a signal at danger at the maximum allowable approach speed and all vehicles permitted to use the line are fitted with tripcock equipment to meet the above requirements for signalling on other portions of their route. C.2 Use as a SPAD Mitigation Measure

Trainstops may be provided on a line of route basis as a means of reducing to ALARP the risk from SPADs.





APPENDIX D -

Reduced Length of Overlap

In track circuit block areas with colour light signals where the level of risk is justifiable, Railtrack may specify that the full overlap be reduced in length. The reduction shall be based on the maximum attainable train speed when trains are running under clear signals at a point 400m (440 yards) on the approach to the signal, in accordance with the following table (Note - the maximum attainable speed may in some circumstances be less than the permissible speed):

Maximum Attainable Minimum Overlap Distance Speed Not Exceeding

15 mile/h 45m (50yards) 20 mile/h 55m (60yards) 25 mile/h 60m (65yards) 30 mile/h 70m (75yards) 35 mile/h 75m (80yards) 40 mile/h 80m (90yards) 45 mile/h 90m (100yards) 50 mile/h 105m (115yards) 55 mile/h 125m (135yards) 60 mile/h 135m (150yards)

Overlap lengths may be extended beyond the above minima where it is appropriate to do so.

In other than track circuit block areas, reduced distances to clearing points may be specified by Railtrack, subject to risk assessment.

In all cases where consideration is being given to the adjustment of overlap lengths, the risk assessment shall take into account the following criteria:

• low adhesion, e.g. known leaf fall sites

• severely falling gradients

• the permitted and assessed speed profile presented to the driver in the final 400m (440 yards) on the approach to a signal

• the presence of any distracting or other factor which may inhibit a driver from complying with the stop aspect of the signal.

Note: Where the maximum attainable speed (being less than the permissible speed) is used in the calculation of the overlap length, the use of attainable speed shall be shown in the notes of the relevant signalling plan including identification of the signals affected. The acceleration data used for determination of attainable speed shall be included in the project records.





APPENDIX E -

Risk Mitigation Measures

E.1 Possible risk mitigation measures which affect Likelihood :-

• AWS equipment (positioned in accordance with GK/RT0016) • “RA” indicators - controlled by the starting signal aspect and in use for all trains (GK/RT0031) • In-cab or platform mounted CCTV display - controlled by the starting signal aspect (on DOO lines) • Improvement in signal sighting (GK/RT0037) • Alterations to signalling (e.g. provision of banner repeating signals) • Use of Driver’s Reminder Appliance (DRA) (GK/RT0091) • Restrictive Interlocking • Timetabling to avoid conflicting moves • Countdown markers which exhibit advisory approach speeds. E.2 Possible risk mitigation measures which affect Consequence :-

• SPAD indicators (GK/RT0031) • Additional AWS • Klaxons • Train Stops (GK/RT0017) • Trap Points • Automatic warning via radio • Alterations to signalling (other than restrictive interlocking). E.3 Possible risk mitigation measures which affect Likelihood and

Consequence:-

• Layout Alterations • Automatic Train Protection • Train Protection Warning System (proposed).





References GK/RT0016 Automatic Warning System (AWS)

GK/RT0017 Trainstops GK/RT0031 Lineside Signals and Indicators GK/RT0037 Signal Sighting GK/RT0060 Interlocking Principles GK/RT0091 Driver’s Reminder Appliance GK/RC0578 Platform Starting Signal Risk Model


Overrun Protection and Mitigation - RSSB Iss 5.pdf · implemented in Track Circuit Block areas.)...

Documents

Transcript of Overrun Protection and Mitigation - RSSB Iss 5.pdf · implemented in Track Circuit Block areas.)...