Thursday, 18 April 2019

Crossrail delay

It may seem impertinent to blog about a peculiarly London problem (though, being the capital, London's troubles have an impact on the rest of England and Wales). However, there is a connection to the aborted GWR main line electrification. When the first intimations of a delay to Crossrail's start date included a mention of signalling, the suspicious mind of a former systems analyst immediately leapt to the conclusion that the designers had gone for a unique new system. So it turns out, as this extract from London Reconnections explains:

There was concern about ensuring trains successfully transitioned between one signalling system and another. Meanwhile, if we understand the situation correctly, there are significant problems just getting the signalling equipment on the trains to talk successfully to both the trackside-based communications equipment and the trains’ computer systems.
Transitioning eastbound is easier than transitioning westbound at Westbourne Park. The more difficult part of transitioning is to successfully communicate with the signalling system the trains is transitioning to. It is somewhat easier to terminate communication with the signalling system the train is leaving.

To aggravate the problem transitioning westbound at Westbourne Park, the signalling system being transitioned to is TPWS+. This is a system based on the standard TPWS (Train Protection and Warning System) used on most of Network Rail, but it includes additional safety protection to bring it on par with ETCS when it comes to safety. One feature of TPWS (and TPWS+) is the intermittent (rather than continuous) nature of the system. This relies on fixed signals protecting discrete sections of track rather than utilising a ‘moving block’ to provide protection between trains. Stepping up from ‘fixed block’ to moving block should be relatively easy to achieve. Think of it, metaphorically, as lava dropping from the top of a lava lamp. The ‘moving block’ starts in a fixed position before moving with the train (protecting it in the rear). Doing it the other way around is probably very hard.

In the central section the signalling system is a bespoke version of Siemens’ Trainguard system, which has been updated with Crossrail in mind. So it should have been upgraded with facilities for transitioning to and from TPWS+ as part of the update. In the other direction (westbound) you are moving into TPWS+ which is an industry standard that one has to adhere to. You can’t simply ‘tweak’ TPWS+ so it works with Trainguard. Even if you could, that would require a derogation that needs ORR (Office of Road and Rail) approval. Such approval is not given lightly.

Part of the reason that there is the problem described above is that this was never the plan. The signalling system west of Paddington all the way to Heathrow was supposed to be ETCS (European Train Control System). It changed to TPWS+ because of the problems of introducing ETCS on the GWR in the necessary time frame. Somewhat ominously but understandably, it took ORR [the Office of the Rail Regulator] two years to approve the change because they needed to be satisfied that TPWS+ was as safe as ETCS on the lines affected.

More ominous still, the time allocated to the system integration phase of Crossrail was decided long before TPWS+ was substituted for ETCS and, to the best of our knowledge, the time allocated was not revised upward to take into account the known (or at least suspected) extra complexity. This does seem a puzzling oversight and one wonders, with the benefit of hindsight, whether it would have been better to stick to the original plan and use ETCS throughout on the Paddington – Heathrow section and accept any delays this may produce, rather than exert so much effort getting TPWS+ installed and working with Trainguard.

To make matters even worse, the change of plan now means you also have another transitioning area that originally was not going to exist. This will need to occur north-east of the Heathrow tunnels on the Airport Branch. In hindsight it is clear how much Crossrail were relying on Network Rail being able to introduce ETCS on the Great Western Main Line (GWML) in good time prior to Crossrail opening – as was the plan. It also shows how apparently unrelated issues such as problems on Network Rail’s GWML modernisation project (some of which were in Wales) have affected Crossrail. As has often been pointed out – Crossrail is good for the rest of the country. Unfortunately sometimes the rest of the country can be bad for Crossrail.

(There is some further stuff about the train-builder not being on the same wave-length as the signalling engineers, but that is straying outside my parish.)

ERTMS/ETCS works, as demonstrated on the Cambrian Line. To drop it from the GWR upgrade was a short-term fix which could have long-term consequences. Whether the new physical signal equipment installed in south Wales is compatible with an upgrade to ERTMS is a question I shall leave to electrical engineers.

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