Moving Block Signalling

This POSTbrief introduces the European Rail Traffic Management System, explains the concept of moving block signalling and discusses its potential benefits for rail capacity, which are likely to vary significantly between routes.

Introduction

Network Rail is developing a programme for the national roll-out of the European Rail Traffic Management System (ERTMS), using European Train Control System (ETCS) Level 2 signalling technology, within 25 years. It is also undertaking work to determine whether ETCS Level 3 technology could be used to speed up the deployment of ERTMS to within 15 years.

Implementing ERTMS with ETCS Level 3 has the potential to increase railway route capacity and flexibility, and to reduce both capital and operating costs. It would also make it possible to manage rail traffic using a moving block signalling approach.

In moving block signalling, trains are given permission to move to a specific position anywhere on the track, as defined by a computer system. This is in contrast to the fixed block signalling used on the vast majority of Britain’s railway (in which trains are granted permission to move to a pre-defined, fixed position — i.e. the next signal).

Moving block signalling effectively maintains a safe ‘envelope’ of empty track around each train, which moves with that train. This envelope can be tailored to match the breaking performance and speed of that specific train, optimising line capacity in different situations. For example, the same track could be used to run lower-speed commuter trains (with shorter stopping distances) closer together, and high-speed trains (with longer stopping distances) further apart.

Although moving block signalling can increase the capacity of railway routes compared to fixed block signalling, in practice there are still constraints, largely due to train braking performance and timetabling restrictions. This means that the increase in capacity provided by moving block signalling differs between routes.

Research in this area is conducted by a range of organizations from across industry, academia and Government. Not all of the results of that work are publically available. This briefing draws on information from interviews with experts from academia and industry, and a sample of the publically available literature.

Summary

Network Rail is developing a programme for the national roll-out of the European Rail Traffic Management System (ERTMS), using European Train Control System (ETCS) Level 2 signalling technology, within 25 years. It is also undertaking work to determine whether ETCS Level 3 technology could be used to speed up the deployment of ERTMS to within 15 years.

Implementing ERTMS with ETCS Level 3 has the potential to increase railway route capacity and flexibility, and to reduce both capital and operating costs. It would also make it possible to manage rail traffic using a moving block signalling approach.

This POSTbrief introduces ERTMS, explains the concept of moving block signalling and discusses its potential benefits for rail capacity, which is likely to vary significantly between routes.

Research in this area is conducted by a range of organizations from across industry, academia and Government. Not all of the results of that work are publically available. This briefing draws on information from interviews with experts from academia and industry and a sample of the publically available literature.

Railway Signalling in the UK

There are two elements to railway signalling:

- train detection — which recognises when a section of track is occupied by a train

- movement authority — which gives a train permission to move to a particular location on the track.

For the vast majority of Britain’s railway, train detection and movement authority are provided via track-side equipment, which creates a fixed block signalling system. Under this regime, signals mounted by the side of the track divide the line into sections, called ‘blocks’. Collisions between trains are avoided by ensuring that only one train is allowed into each block at once (Box 1).

Box 1. Fixed Block Signalling
Figure 1: Blocks are defined as the distance between two signals. Trains are prevented from colliding by ensuring that only one train is allowed into a block at once.
In fixed block signalling, movement authority is provided by signals that instruct train divers on whether to stop or to progress through a block. Movement authority is given to a fixed point on the track — i.e. the location of the next signal. Train detection equipment, for example electrical circuits built into the track, can detect whether a train is occupying a particular block. Other track-side equipment (known as interlocking) analyses information from the train detection equipment to determine whether a block is available for a train to move into, before the signals can give a train permission to move.
Typically, the lengths of the blocks on a particular route are chosen to safely accommodate the fastest trains with the longest stopping distances. Stopping distances vary between trains because of a number of factors, such as the speed of the train, the age of the brakes and the weight of the freight being transported. Fixed block signalling therefore prevents better-performing trains with shorter stopping distances from travelling closer together. In some cases, block lengths are chosen to optimise capacity for the trains that use the line most frequently. Trains with longer stopping distances can still use the line, but are run at a reduced speed.

The European Rail Traffic Management System

In 1996, the EU agreed that the European Rail Traffic Management System (ERTMS) should become standard for all high-speed lines in Europe. This was later extended to the conventional European rail network. ERTMS is a signalling and train control system that is intended to facilitate cross-border traffic across Europe; to enhance safety, reliability and capacity; and to help reduce the cost of running and maintaining the railway.[1],[2]

It comprises:

- a signalling, control and train protection system known as the European Train Control System (ETCS)

- a radio system (currently used on railways throughout Europe) that provides voice and data communication, known as GSM-R

- a traffic management system to optimise train movements through the ‘intelligent’ interpretation of timetables and train data, known as the European Traffic Management Layer (under development)

- a set of operating rules, known as the European Operating Rules (under development).[3],[4]

ETCS can be deployed at a number of different levels (Box 2). At present, use of ETCS in Great Britain is limited. Level 2 has been in use on the Cambrian Line since 2011 and there are plans to deploy it on sections of the Thameslink and Crossrail routes.[5],[6],[7] Network Rail is also running trials with ERTMS technology suppliers on the Hertford Loop (north of London).[8]

ETCS Level 3 is under development. Some standards are still to be agreed, for example for a train-based integrity system to check that freight trains are complete (i.e. that a wagon has not become detached), which would have traditionally been done with track-based train detection equipment.[9],[10] Not all of the equipment required for ETCS Level 3 is commercially available.[11] A pilot system with most of the characteristics of ETCS Level 3 has been developed in Sweden on a low capacity rural line: the “ERTMS Regional”.[12],[13],[14] In addition, systems with characteristics similar to ETCS Level 3 are operating in pilot projects in Kazakhstan on very low capacity main line routes.[15],[16]

In Great Britain, Network Rail originally proposed a national roll-out of ERTMS with ETCS Level 2 technology, primarily on the basis that it would provide a more efficient way of renewing signalling assets.[17] This plan would have involved ETCS Level 2 being installed when existing signalling schemes came up for renewal, and was expected to take up to 50 years to complete.[18] Under the Digital Railway Programme, Network Rail is now devising a plan to speed up the national roll-out of Level 2 ETCS signalling to within a nominal period of 25 years.[19] It is also exploring whether ETCS Level 3 technology could be used to speed up national deployment to within 15 years.[18]

Box 2. ETCS Deployment Levels
ETCS can be implemented at different levels, depending on how the route is equipped and the way in which information is transmitted to the train.
Level 1 — Trains are equipped with ETCS technology and ETCS is installed at the track-side. Equipment on the track is responsible for train detection and intermittently transmits movement authority to the trains.
Level 2 — As level 1, except all trains receive an almost continuously updated movement authority from a radio control centre via GSM-R radio transmissions. Track-side equipment is still needed for train detection.
Level 3 — ETCS replaces both the track-side signals previously used to provide movement authority and track-side train detection equipment. Standards and technology for Level 3 are under development.

Network Rail report that modelling they have conducted on the South West Mainline into Waterloo suggests that ETCS Level 3 — used in combination with automatic train operation (to assist in driving the train) and a traffic management system — might deliver up to 40% more capacity on certain sections than at present, at around 30% lower cost than conventional line construction.[20],[18]Cost and time savings are expected to come from the simpler, more automated design process and the elimination of track-side infrastructure, among other factors, but will depend on the architecture of the system adopted.[13] Other benefits would include greater flexibility. For example, running slower commuter trains closer together at peak times, and faster trains (that need to be spaced further apart) at off-peak times. Making enhancements, such as the addition of junctions, should also be cheaper.

Operating at ETCS Levels 2 and 3

For both ETCS Levels 2 and 3, track-side signals are replaced by displays inside the driver’s cab. Trains monitor their position using on-train sensors and balises (electronic beacons) fixed to the track. They report their location and speed to a radio block centre (RBC) via radio transmissions. The RBC receives information from all trains in a particular operating area, allowing it to map the traffic on the network. It checks that a route is clear, reserves it for a particular train and then transmits a movement authority (and other information) directly to the driver’s cab (Box 2).

Trains are in near-constant contact with the RBC, minimising the time delay between a section of track becoming available and a driver being given a new movement authority. On some sections of track, earlier notification that the track ahead is clear could reduce the amount of time spent braking or speeding up unnecessarily, increasing capacity compared to that achieved with current signalling.

The key feature of ETCS Level 3 is that train detection no longer relies on track-side equipment (unlike Level 2). Instead, the safety-critical task of determining whether a section of track is occupied is done by the on-board computer and RBC. Level 3 is expected to require a new set of operating rules, which have yet to be agreed.[21],[22]

ETCS Level 3 technology would allow a railway to be operated in either a “moving block signalling” (MBS) or a “virtual” fixed block signalling mode. The mode of operation would depend on how the system was configured.[23],[13]

Moving Block Signalling

In a MBS mode, instead of blocks being defined by fixed points on the track, as is the case with fixed block signalling (Box 1), they are defined by a computer system. Each train determines its own location and reports it to the RBC. The RBC calculates the safe movement limit of each train in real time, based on information that includes the known position of all other trains in the area and track conditions.[13],[24]

Instead of trains being given permission to move to a specific signal or stopping point, they can be granted permission to move to a position anywhere on the track. As the train in front clears more of the track and reports its progress to the RBC, the movement limit for the train behind is continuously extended.[25] This effectively maintains a safe ‘envelope’ of empty track around each train, which moves with that train. MBS allows the envelope to be tailored to match the braking performance and speed of that specific train, optimising line capacity in different situations.[13] For example, the same track could be used to run lower-speed commuter trains (with shorter stopping distances) closer together and high-speed trains (with longer stopping distances) further apart.

In principle, MBS makes it possible to operate with the smallest possible block lengths (just long enough to accommodate one train plus the distance it would need to stop safely). This allows the maximum number of trains possible to be run on a route. Block length might also be reduced in a fixed block signalling system, by physically reducing the distance between signals. This increases the number of blocks, and therefore the maximum number of trains that can be run on a track. However the fixed nature of the signalling system would mean that the track would only be optimised for trains with a particular stopping distance. Other trains might need to run at reduced speeds to ensure that they stop safely. In addition, reducing block length on an existing fixed block system requires the installation of extra track-side equipment, which can be costly.

Moving Block Signalling and Capacity

Although MBS can increase the capacity of railway routes compared to fixed signalling systems, in practice there are still constraints, largely due to train braking performance and timetabling restrictions. This means that the increase in capacity provided by MBS differs between routes. Junctions and stations can also create constraints that MBS cannot address. Trains may need to wait for a junction to be re-routed, or for another train to vacate a station platform, before being given permission to proceed.

MBS works best on lines where trains have similar stopping patterns and performance characteristics, and move with similar speeds in the same direction. The majority of MBS systems implemented to date have been on metro lines, using alternative communications-based train control systems to ETCS (Box 3). These include the Docklands Light Railway, Vancouver’s Skytrain and underground lines in Singapore and Beijing.[26],[27],[28] The use of MBS on non-metropolitan railways has been mainly limited to industrial applications (mostly mining) and the very low capacity lines in Kazakhstan.[29]

In 2010, the Transport Research Laboratory carried out modelling of different types of UK railway route. Their study suggested that ETCS Level 3 with MBS could reduce headway (the time interval between two trains following each other) by up to 10–20% compared to ETCS Level 2.13 However, the inclusion of freight trains on the same routes reduced this by 5–10%. The study indicated that Level 3 could deliver major capacity benefits on regional routes that are currently constrained by long sections of fixed blocks (for example the Fen Line), as well as in some high-capacity urban applications (such as the South-West Main Line). In most other scenarios, route capacity was found to be constrained by other factors, which conventional signalling had already been optimised to match.

Virtual Fixed Block Signalling

As an alternative to MBS, ETCS Level 3 could also be used in a “virtual” fixed block mode. In this case, the system simply mimics the operation of a fixed block system. Discussions are underway at both national and European levels about whether ETCS Level 3 should be operated in a moving block or virtual fixed block mode.[13]

[1] European Commission website, accessed 13/4/16

[2] ERTMS Online website, accessed 13/4/16

[3] International Union of Railways website, accessed 13/4/16

[4] Your guide to European Rail Traffic Management System (ERTMS), ERTMS Online, 2015

[5] Hitachi onboard ETCS technology successfully operating with Network Rail track-side system, Hitachi news release, 10/6/13

[6] ERTMS.net website, accessed 13/4/16

[7] Crossrail ETCS GRIP 1–3 Options Analysis, Vertex Systems Engineering, 2015

[8] Network Rail website, accessed 13/4/16

[9] Reliable Data Systems International website, accessed 13/4/16

[10] Operating rules pose a major challenge to ETCS implementation, Railway Gazette, 26/2/08

[11] International Union of Railways website, accessed 13/4/16

[12] Level 3 : From High Speed Vision To Rural Implementation, Institution of Railway Signal Engineers International Technical Committee, 2011

[13] ERTMS Level 3 Risks and Benefits to UK Railways Final Report, Transport Research Laboratory, 2010

[14] Swedes unveil first ETCS Level 3 application, Railway Gazette, 25/4/12

[15] Bombardier Transportation (Signal) Ltd Commissions First Radio-based Main Line Train Control in CIS, press release, 5/5/14

[16] IRSE Matters, Institution of Railway Signal Engineers, IRSE News, Issue 215, October 2015

[17] ERTMS National Implementation Plan, Department for Transport, 2007

[18] Digital Railway Discussion Pack, The Digital Railway Programme, Network Rail

[19] Personal communication with Andrew Simmons, Chief Systems Engineer for Digital Railway, Network Rail

[20] Wessex Route Study, Network Rail, 2015

[21] Elusive ETCS Level 3 still a long way off, D. Briginshaw, International Railway Journal, 7/3/16

[22] ERTMS — A reality check. Rail Engineer, C. Kessell, 28/8/15

[23] Institution of Railway Signal Engineers (Hong Kong), Newsletter Issue 06, March 2010

[24] The ERTMS/ETCS Signalling System, M. Palumbo, 2014

[25] Moving Block in Communication-Based Train Control: Boon or Boondoggle? B. Ede, 2006

[26] Urban Signalling for the Future, Now, Timothy Ludikar, IET, 2010

[27] Driverless Railway Systems — A Case Study of Singapore MRT System, Yee Boon Cheow, Land Transport Authority of Singapore

[28] Beijing opens world’s longest CBTC-based metro line, Siemens, press release, 2/1/13

[29] Bombardier Site Fact Sheet, Gothenburg, Sweden