The 73-mile Elizabeth Line – running from Reading in the West, through to Bond Street, Farringdon and Canary Wharf in the heart of Central London, and out to Shenfield and Abbey Wood in the East – is complete. Tunnels have been drilled deep under London and brand new stations have been built. But the intricate lines of code needed to make Crossrail’s software and signalling systems run smoothly aren’t quite there yet.
Let’s be clear, Crossrail is a hugely complex project. It involves linking two Victorian-era lines to the east and west of London with a sophisticated new underground track that cuts through one of the oldest and busiest cities on the planet. You might think the sheer scale of Crossrail would be the biggest challenge, but you’d be wrong.
The 13-mile long metro tunnels that connect the Great Western Main Line and the Great Eastern Main Line sink to a maximum depth of 40 metres, and they have a diameter of 6.2m. It took eight 1,000 tonne tunnel boring machines three years used to dig them out. But that’s all done and dusted. Now, with everything else complete, the signalling systems that make everything work together in harmony still need to be fine tuned and knitted together.
Building software and signalling systems that enable the new metro track, the Great Western Main Line, and the Great Eastern Main Line to talk to each other, and to the new trains, is proving to be a major challenge for Crossrail, which wants to run 24 trains an hour at peak times between Paddington and Whitechapel, and serve 200 million passengers annually. “There’s a big mish mash of technologies on both of those [old] railways,” says Colin Brown, Crossrail's technical director.
If the signalling systems don’t work properly, then trains will end up stuck in tunnels while they wait for a platform to become available, or, in a worst case scenario, trains will crash into each other.
Different parts of the Crossrail route are going to rely on different signalling systems. The Great Eastern and Great Western railways both have a decades-old signalling system called AWS (Automatic Warning System). However, after a series of accidents in the 1990s, AWS was coupled with a new system called TPWS (Train Protection Warning System). Now used across the UK, AWS/TWPS provides train drivers with a cab warning of the indication of the next signal. If a driver goes through a red light then AWS/TWPS will automatically apply the brakes.
In order to merge these old UK systems with new technologies that will last for up to another 100 years, Crossrail is working with Siemens and Bombardier on an interoperable rail system.
“We’ve chosen a technology called ERTMS (European Rail Traffic Management System) because it’s future proofed for the Great Eastern and Great Western railways, and also because it’s a modern safe system of separating the trains,” says Brown. “The challenge of making ERTMS work in the UK is that you have to make it compatible with the legacy signalling systems: AWS and TPWS.”
The real challenge has been making ERTMS work underground. In order to do this, Crossrail has had to incorporate a radio system, called CBTC (Communications-Based Train Control), which is developed by Siemens. A similar system is used on the Jubilee, Victoria, and Northern lines and the DLR.
“CBTC is what you would see on any mass transit system,” Brown says. “It’s a high-precision signalling system and it’s high-precision in that it can manage stopping distances incredibly accurately to align doors and it’s got additional levels of safety and interfacing with things like tunnel ventilation and timetabling. And how the service can recover from problems that happen in a metro scenario.”
In total, there are three signalling systems (CBTC, ERTMS, and TPWS) that the new trains have to be able to run on. “That means we’ve got a very complicated train,” says Brown. The number of acronyms alone hint at how gnarly and complex signalling has become for the project. Crossrail is now testing these systems and the associated software to the nth degree as there’s no room for error. A small mistake on the network could turn out to be disastrous, even deadly.
“We have 200 test cases which we are working on completing successfully before we enter into passenger running,” Brown says. Software and systems testing can be a long, arduous process, says Mark Moloney, a test analyst at Contigo Software, who has scrutinised stock broking systems, energy trading software, and apps used to order food and drinks in restaurants. The aim is to check and make sure that all the software developer’s “wonder code” is up to scratch and does what it’s designed to do.
“Testers try and break the new stuff by acting like an end user that’s never used the system before,” says Moloney. They write a number of test scripts and put them together to create an end-to-end journey that depicts how someone might open the app, use it in a particular way, and get to an end point. The testing script is run automatically (with other software) or manually (by a human). Testers also perform “exploratory testing,” which is “someone just doing crazy shit to find out what will happen,” according to Moloney. Testers also have to run regression tests to check that the new code hasn't broken any of the code that existed before.
Crossrail says that it is currently doing what it calls dynamic testing on Bombardier and Siemens software and data at the Crossrail Integration Facility in Chippenham. The facility is designed to allow Crossrail to mitigate any errors or defects early, before being tested on the railway.
Once the software has worked off-site, it’s then delivered to the central section of the Crossrail track to be tested with real trains and collect evidence of any software bugs in the train control system. Crossrail started running tests in January 2019 with one train, before moving up to two trains (one in each tunnel), and then to four trains.
Crossrail is not only late, but it’s also running well over budget. The target opening date has been pushed back to some point in 2021 and costs could now soar to £18.25 billion, which is over £2bn more than the original budget of £15.9bn.
Businesses, homeowners, and developers that have moved closer to a Crossrail station in order to reap benefits are growing impatient. And when they heard Crossrail had been delayed again, due to software and signalling issues, they were less than impressed.
“The two critical paths for the project remain software development for the signalling and train systems, and the complex assurance and handover process for the railway; both involve safety certification for the Elizabeth Line,” Crossrail chief executive Mark Wild said in a statement. “These must be done to the highest quality standards to ensure reliability of the railway from day one of passenger service.”
The intricate and complex task of knitting together old infrastructure with new, and delivering what would be one of the busiest and most efficient metro lines in the world, is no small task. But for Crossrail, delays are costs are already spiralling out of control. “The Elizabeth Line will open as soon as practically possible in 2021,” Wild added.
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This article was originally published by WIRED UK
