Looking beyond the hype on the digital railway

Passengers on crowded platforms may not accept that their railway is running at capacity when there are no trains to be seen.

This is because the signalling system must ensure that trains are separated by their braking distance. This is 2km for trains moving at 200km/h.

The distance between trains is further increased on mixed-traffic railways carrying express, stopping and freight trains, which significantly reduces line capacity. In contrast, on metro railways all trains have the same operating characteristics and there are few junctions. Hence, metros can carry 30 trains per hour (tph) or more, while mixed-traffic railways do well to achieve 12tph. 

The 40,000 signals on the mainline network are controlled by various generations of mechanical, electrical or computer interlockings which provide safe train separation. The age profile of this infrastructure is such that by the mid-2020s renewals will be four times their current volume. This presents a delivery challenge, for which a new approach is required. 

Computer in the cab

Eliminating lineside signalling is one such approach. This is possible by using the European Traffic Control System (ETCS) level 2 to bring signalling into the driver’s cab. This replaces the driver seeing signals to get a movement authority (MA) over a long block to the next signal with an in-cab display of maximum permitted speed. This is calculated by an on-board computer which considers speed, gradient and braking and the MAs it receives by a secure radio link from the interlocking. As ETCS can have smaller blocks, its more frequent MA update provides some capacity benefit.

ETCS level 2 uses existing train detection systems. Lineside signals are only required if some trains are not fitted with ETCS. Soon many routes will only have ETCS-fitted trains as government policy is that the many new trains now being delivered will be equipped with the system. It is also policy that future signalling schemes should be digital ready. This means designing infrastructure with more signalling blocks. 

In theory, ETCS level 3 offers significant capacity benefits as it eliminates the need for lineside track detection systems. Instead the train location, determined from passive electronic beacons and odometry, is transmitted to the signalling interlocking which transmits back the MA. It offers moving block signalling, allowing reduced headways between slower-speed trains. However, owing to significant practical problems, level 3 has yet to be implemented on any mixed-traffic railway. 

While not part of the signalling system, Automatic Train Control (ATO) also offers capacity benefits by ensuring that all trains have the same braking and acceleration rates. In a world first, Thameslink trains introduced ATO on an ETCS level 2-signalled route in 2018.

Another aspect of the digital railway is TMS (Traffic Management System) which monitors each train against its timetable, forecasts train times, identifies conflicts and presents running data to assist signallers to take optimum routing and control decisions or control automatic route setting. 

TMS also calculates the ideal speed of the train to help ensure it travels smoothly through the network. It has been estimated that TMS can reduce delay minutes by up to 7%.

Promising too much

Throughout the world, digital signalling has significantly increased the capacity of metro trains. Yet the same cannot be said of a mixed-traffic railway for which infrastructure configuration is the main capacity constraint. Hence there is no basis for claims that digital signalling will provide a 40% capacity increase, and so eliminate the need for infrastructure enhancements.   

Digital signalling offers many benefits. But on the mainline railway its capacity benefits should not be overstated, and it is unlikely to replace the need for physical capacity-enhancement work.

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