On 10 May 2018, the Secretary of State for Transport, Chris Grayling, and Mark Carne, chief executive Network Rail, supported by David Waboso, managing director of Group Digital Railway, launched the Digital Railway Strategy.

With digital train control already a reality on the Thameslink core through London Bridge and on Crossrail, they announced that, in the five years to 2024, the industry is planning to introduce more of the same across the Pennines, on the southern end of the East Coast main line into London King’s Cross and onto some of the major commuter routes that feed London Waterloo. Within 15 years, the aim is to see 70 per cent of journeys benefit from digital railway technology.

To enable this transformation, they also announced a commitment that all new rolling stock must come from the factory digital signalling ready and that all conventional resignalling must be provided ‘digital railway ready’. But what does that mean, and what is digital signalling?

Digital versus Analogue

It could be argued that all signalling is ‘digital signalling’. The first mechanical interlockings introduced a way in which trains were less likely to run into each other by using interlocked levers connected to points and signals, such that rules for train movement were enforced. The system drove points that were either in one position or another and signals that told drivers to go or to stop – a two-state binary, or digital output.

The next stage of signalling technology involved electrically operated relays. Effectively imposing signalling rules via long strings of relays processing ONs and OFFs. For example, a series of relays are connected to points, track circuits and signals. If one relay was energised, and another de-energised, with either of two other relays de-energised, then another relay (controlling a route) could be energised. The great thing with relays is that the logic could be designed so that if any relays fail, the system ‘fails safe’.

Railway signalling developed via Solid State Interlocking to modern computer-based interlockings, which use processors and software to replace the relays and mechanical interlocking logic. The evolution of technology has always provided a similar safe ‘digital’ output to a driver, but on different platforms.

It could even be argued that modern signalling is less digital and more analogue than previous systems. In-cab signalling displays give drivers information about how far and how fast the train can safely go – not simply the status of the next signal. This is provided in terms of analogue value positions to the nearest metre and speeds to the nearest km/h.

However, today the term ‘digital signalling’ is used to describe computer-based interlockings digitally connected to ‘objects’ (points, signals, fringes, trains) and the way they are controlled, connected or monitored, so modern signalling is actually ‘networked signalling’ or ‘communications-based’ signalling. Central or distributed interlocking processors control networks of objects using secure digital messages transmitted at high speed, which may include radio to provide movement authority to trains.

The Digital Railway Strategy is to provide in-cab signalling using the European Train Control System (ETCS) to allow trains to run closer together in greater safety and with more reliability. ETCS, when married with traffic management, can dramatically reduce knock-on delay – which is currently the largest single cause of train disruption.

The world’s first full-scale fixed-block Automatic Train Operation system (ATO) system was installed on the Victoria line in London 50 years ago and telecoms engineers have been using digital multiplexing data transmission for decades. So, as Mark Carne and David Waboso both made the point at the launch of the Digital Railway Strategy, this is not new technology and the industry can’t go on installing inefficient colour-light signalling systems if it wants to improve capacity, reliability and safety.

Digital Railway

The Digital Railway solution involves a number of digitally enabled technologies to improve train performance and provide increased capacity on the GB railway network, with the three main components being:

ETCS and GSM-R radio to communicate between the train and the Radio Block Control (RBC), with ETCS also providing Automatic Train Protection (ATP), which enforces obedience to signals and speed restrictions by speed supervision;

Traffic Management System (TMS) – a signalling control system to optimise the throughput and timetabling of trains, particularly in times of train disruption;

Driving support technologies – Connected Driver Advisory System (C-DAS) advises the driver on the optimum train speed profile whilst Automatic Train Operation (ATO) systems can deliver elements of automated driving.

All three systems can be used together or individually to suit the requirements of different routes and railway traffic patterns.

Network Rail is drawing up the deployment and implementation plan that will be used for the majority of signalling and control system upgrades. However, until the Digital Railway components are all ready to be deployed, with trains fitted with the relevant on-board equipment, it will be necessary to continue to install traditional colour light signalling to maintain safe and reliable asset performance.

Signalling renewals, alterations and enhancements will therefore need to be implemented in a way that allows the subsequent introduction of Digital Railway components with minimum alterations, so that the signalling system is ‘Digital Railway Ready’. Network Rail has produced a standard that will enable any signalling works, undertaken in advance of a future Digital Railway deployment, to be upgraded to ETCS with minimum disruption and cost.

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Underlay and Overlay

An ETCS Overlay scheme will involve overlaying the ETCS on to the existing layout without significant alterations to the train detection sections or signal positions. The train detection sections will be positioned to suit the optimum lineside signal locations. It is envisaged that an Overlay solution will be used where dual running is required (so lineside signals will be retained for a number of services that are not ETCS fitted) or where the existing train detection sections provide the headway performance required.

An ETCS Underlay scheme will be optimised to simplify the introduction of ETCS without lineside signals. This involves assessing train detection sections to suit the final train stopping positions of the ETCS solution in any interim design. Any lineside signals are positioned according to the final train detection sections (unless this will compromise the conventional signalling arrangements) or existing signals retained to suit the operating requirements of the non-ETCS fitted trains.

All new interlockings must be provided ETCS compatible. This means all interlockings will be modern computer-based interlockings (CBI) that have been designed to work with an ETCS RBC. The interlocking must be provided with a communications link, protocol and software capability to communicate with an RBC.

Scheme designs

All signalling renewals or enhancements will be designed and constructed to allow for reduced intervention for subsequent ETCS deployment (such as a staged introduction of ETCS and its functionality). The outline signalling scheme design will need to take into account the subsequent deployment of ETCS and be developed to enable a simple upgrade path without further interventions or costs. This will need to take into account such variables as train detection limits and level crossing interfaces.

The scheme layout design for an ETCS underlay scheme will need to be optimised for ETCS, as the full benefits will only be gained if the design is optimised from the initial scheme plan stage. Interfaces between the level crossing equipment and both the conventional and ETCS control systems will need to be assessed.

Interface specifications for signalling fringes will need to take into account future migration to ETCS and to include an assessment of the interlocking and RBC fringe requirements. The locations where transitions between conventional signalling and ETCS will take place will also need identifying.

Interlocking requirements

The interlocking of any signalling renewal or enhancement will require, in addition to the normal 10 per cent spare capacity, to have a further allowance calculated to support final ETCS functionality. This will be established on a project-by-project basis.

The intention will be that the RBC can be attached to the interlocking with no requirement for a change or upgrade to the hardware (other than enabling the interface). The additional capacity will provide for future interlocking expansion to support a Digital Railway and be calculated based on the standard ETCS functions, which includes signalling to allow all reasonable movements in both directions.

Application data

The application design will need to support an increase in capacity and/or flexibility with minimal change to the data. In order for the future Digital Railway solution to deliver enhanced benefits, it might be necessary to alter existing infrastructure arrangements or create additional block sections. Where future infrastructure changes or additional train detection sections are proposed for ETCS capacity/flexibility enhancements, the conventional signalling will need to be designed to accept infrastructure modifications and the introduction of the extra train detection sections.

The objective of the Digital Railway Strategy is to create repeatable data that can be proven generically and allow the automation of data production. This will require a modular approach to support repeatable design, automated deployment of designs, progressive introduction and removal of functionality and code, together with testability.

The data will also need to take into account the ETCS requirements for emergency release of track locking of points and the emergency release of route locking, swinging overlaps, active level crossings, and pre-defined ETCS shunt areas. Any ETCS-specific application data included in the interlocking prior to the introduction of ETCS must not impact on the overall safety or functionality of the interlocking.

Train detection

The use of axle counters is the preferred method for train detection as the upgrade from ETCS-ready conventional re-signalling to a full ETCS system may require alterations to the axle counter train detection sections which are easier to modify than track circuits. The axle counter trackside architecture should therefore be designed to simplify future modifications.

Altered or additional train detection sections may be required when upgrading to ETCS. This includes optimising train detection sections to match stopping points, rather than colour light signals. The axle counter architecture and interface to the interlocking will need to be designed so that alterations are minimised.

An ETCS Underlay project will be designed to move to ETCS with a minimum of alterations (as the final ETCS layout is already known). Therefore, the track sections can be designed to suit the final ETCS layout to meet the ETCS train stopping positions and headway performance layout.

Migration from ETCS Level 2 to a future level 3 system may require alterations to the axle counter train detection sections, particularly on plain-line sections between junction areas. The axle counter trackside architecture will therefore be designed to allow recovery with minimum alterations to the signalling system.

If ETCS Level 3 becomes available after the re-signalling, then upgrading direct to Level 3 instead of migrating via Level 2 may be possible. In this scenario, the train detection system may not be required, apart from train detection sections over points. The axle counter architecture and interface to the interlocking should therefore be designed so that recovery alterations are minimised.

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Lineside equipment

To simplify and reduce the cost of decommissioning and recovering the lineside signals and any signage, the lineside equipment will need to be designed to simplify recovery when ETCS is implemented. For example, separate signal module and axle counter housings would allow for easier removal of signals.

On an ETCS Underlay re-signalling scheme, the lineside signals and associated AWS, TPWS and overlap lengths can be positioned to suit the ETCS optimised train detection sections, to avoid alterations to the train detection when the signals are recovered.

Where conventional signal positions are not aligned to the final ETCS (optimised) train detection sections, a range of options, including interim alterations to final train detection arrangements, can be considered and subject to risk assessment. Alternatively, selected or all existing conventional signals could be retained to accommodate non-ETCS fitted trains.

When migrating to ETCS Level 2 (retaining signals) and ETCS Level 3 (no lineside equipment, just train detection), to simplify and reduce the cost of decommissioning and recovering equipment housings, or the redundant equipment within equipment housings, the housings will need to be designed to simplify recovery. This could be by using separate equipment housings or by making the equipment easy to disconnect and recover.

Telecommunications

An assessment of all the telecoms systems will need to be undertaken during any project development to establish GSM-R and fixed telecoms requirements, including access layer capacity, coverage and any constraints.

GSM-R currently provides a circuit-switched data connection from the RBC to the train ETCS system to communicate a movement authority. The second-generation radio circuit-switched data technology used in GSM-R means a radio connection is required to a train, even if no data or very little data is being transmitted. This is an inefficient method of data transmission and, as ETCS is rolled out throughout the country, GSM-R may not be able to provide the capacity required.

Therefore, at some point in the future, upgrading GSM-R to packet-switched data in order to provide additional data capacity, or completely replacing it with 4G/5G LTE technology, may be required. Given the age of GSM-R, which is a 2G system, it is likely that replacement with LTE/4G/5G or another form of radio data technology may be required.

The digital railway is a long-term programme that will that will continue until 2030 and beyond. It is therefore inevitable that, in the few first years of the programme, there will be conventional signalling renewals as well as early ETCS digital railway deployments. In addition, until all trains are fitted with the required equipment, on some lines there will be a requirement for both ETCS and colour light signalling.

The ‘Digital Railway Ready’ standard takes account of these scenarios by requiring engineers to consider ETCS digital signalling as they prepare conventional schemes. It is just one indication of the growing momentum of digital railway delivery.


Read more: Understanding the commitment to the Digital Railway