The railway is full; how can we increase its capacity?

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Mohamed Samra
Mohamed Samra

Mohamed Samra, Research Fellow in BCRRE, introduces how Moving Block and Virtual Coupling gives us the opportunity to increase the capacity of the railway.

Rail transport is already an efficient mode of transport that brings great economic, social, and environmental benefits. The railways around the world are facing growing demand to transport more people and freight. However, it is expensive to build new tracks, so the current approach is to maximise the utilisation of the existing infrastructure to run more trains. To achieve this objective, more attention has been paid towards developing signalling systems and my colleagues in BCRRE and I are 18  months into a 2-year project which is bringing together technologies and concepts for moving block and virtual coupling.

Signalling Systems

To put this into context, let’s remind ourselves why we need signalling systems and do some revision on signalling systems.

Trains are heavy and high operating speeds make it challenging to stop them within the sighting distance of the driver. Of course, trains are guided on rails, making it impossible for them to change direction or pull over to avoid a certain incident. It is the responsibility of the signalling system to ensure that trains run with safe separation (i.e. avoiding any possible collision) and with maximum efficiency for train operations (i.e. following correct route and the proper timetable).

Fixed Block Signalling

In a traditional, fixed block, system (e.g. ETCS Level 1 and Level 2), the network is divided into sections, known as “blocks”.  Normally, only one train is permitted in each block at any one time. The concept of Fixed blocksindicates that this section of the track is located between two fixed points. It is always challenging to size these blocks so that they achieve the optimum headway for improved network capacity and safety.

In the case of a three-aspect fixed block signalling system, the overall headway t can be expressed as follows:

Where D1 is the distance between the second train and the first signal. This distance allows the driver to notice the signal ahead clearly. (D2 + D3) and (D4 + D5) are the block distances between signal 1 and signal 2, and signal 2 and signal 3, respectively. (D4 + D5) is added to represent the braking distance needed by train 2 to stop by the red signal 3. D6 is the overlap distance between signal 3 and train 1, D7 is the length of the first train, and v is the speed of the second train.

In the fixed block system, the block size does not take into consideration train length or speed, which results in long headway and reduced network capacity. For instance, if the block length in the above figure is 100m but the train’s length is only 50m we are already wasting space. Also, the block size is fixed for trains travelling at various speeds without considering the various braking behaviour of each train. Slower trains tend to take less time and distance to stop compared with faster trains.

Moving Block Signalling

The moving block concept, shown in the following figure, was introduced to enhance the line headway, and therefore the overall capacity by eliminating these unneeded fixed separation distances. Now, in a moving block system (e.g. ETCS L3), the track is being dealt with as a continuous guideway rather than being divided into smaller chunks.

The overall headway  can be expressed as follows:

Where D1 represents the safety separation distance between the moving trains, and is a function of the second train’s speed. D2 is the braking distance of Train 2, while D3 is an additional safety margin. Finally, D4 represents the length of the first train. Unlike the fixed block signalling system, D1 and D2 are not static but rather depend on the train’s speed and acceleration/deceleration class, which drastically enhances the overall line capacity by eliminating the unneeded separation distances.

Virtual Coupling Concept

In a moving block system, the headway distance is highly dependent on the train speed and braking distance. As the braking rates are limited by adhesion between the wheel and rail, the separation distance increases considerably with speed. The virtual coupling concept (i.e. ETCS L4) aims to improve the network capacity further by running trains much closer together. The virtual coupling is built on the assumption that the leading train will not halt instantly, however it will break gradually until it stops. If the following train has a similar braking rate as the leading train, the two trains can run with a separation distance that is less than the full braking distance of the following train. A communication link between the two trains is essential to ensure the braking behaviour of the leading train is reported continuously to the following train to act accordingly and maintain a separation distance at all times, as shown in the following figure. In other words, the headway in a moving block system is based on absolute braking distance between trains, while the virtual coupling enables a relative braking distance by allowing the following train to incorporate the braking behaviour of the leading train. The virtual coupling is quite similar to driving a vehicle on a road, where the driver reacts to the brake lights of the vehicle in front and starts braking gradually until both cars become stationary together.

There are a number of challenges associated with virtual coupling, most notably related to safety: at junctions; having reliable communication between the trains; and avoiding collision due to different braking behaviours.

MovingRail Project

This is the theme of the BCRRE MovingRail project:  a multidisciplinary EU-funded project which brings together partners from across the UK and Europe.

We are analysing the application of moving block and virtual coupling signalling, with a focus on increasing line capacity and reducing wayside life-cycle costs. The project breaks down into 7 key objectives: (i) identify operational procedures for moving block and virtual coupling signalling systems; (ii) validate moving block operational and engineering rules; (iii) define improved strategies and methods for testing of moving block signalling systems; (iv) assess compliance of radio-based communication structures and system architectures; (v) identify suitable automated vehicle communication technologies; (vi) identify potential markets for virtual coupling; and (vii) provide a roadmap for the introduction of virtual coupling.

Full details of the project and its results to date can be seen on the project website [] and please join the conversation about moving block, virtual coupling and solutions to increase railway capacity.

Mohamed Samra can be reached at

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