I'm wondering why don't they use GPS as a redundant data source too.
The issue is not the idea, it's that railway signalling systems have to be an interoperable, multi-vendor, tested and accredited pile of little ideas like this, that have infinite numbers of ways to go wrong and have corner cases by the metric ton. And it's not like coding a website, where you can just ignore standards and focus on your case. "Your" rails need to support other people's trains. "Your" trains need to run on other people's rails. Even in a fully closed system with your trains on your rails... who's going to be your equipment maintainer and supplier for the next 30-50 years of their operational life?
Read this article and notice how often cost and project overruns are mentioned:
https://www.railengineer.co.uk/etcs-preparing-the-east-coast...
> The use of ETCS is driven by deliverables, not the technology but concerns exist as to why it is costing so much. In Europe, the success of ETCS is driven by country mandate and a system authority which helps to create a stable ongoing work plan.
> There is also the question of strategic safety versus functional safety, namely fundamental versus site specific. Over-specified ETCS undermines the strategic safety and functional over scope and so drives unaffordability since the accreditation inflates the cost.
> Cost must be driven out of ETCS if it is to be the preferred system for ongoing renewals. Despite interoperability requirements, there is risk in always using the same supplier. There is no common way of delivering ETCS. Having it as an overlay system will always be expensive and not beneficial to the business case. Lessons have been learned from the application of ETCS to the Cambrian route, the Thameslink central core, and the Northern City line from Finsbury Park to Moorgate but all of these have only one type of rolling stock in day-to-day operation. Equipping a mixed traffic railway as in the ECML makes it much more difficult.
... via mobile data or WiFi, you mean. Whether it uses GPS, Galileo, track measurement points, or even "driver calls the control centre to update their position" doesn't have much to do with it. What I assume you mean is redundancy so that two or more systems can communicate train positions independently, where the more relevant problem is how to get the data across in a timely manner. The driver of the stationary train called in their position and it wasn't used: it's a communication problem, not a "where am I" problem. Idk why the focus on GPS and using GPS for communication (which GPS can't do, and newer systems only do at very low speeds for emergency communication) but finding one's position is usually not the problem
1. Planes can determine their altitude. Trains don't know which track they're on. You'd have to re-engineer existing signalling systems designed around the inverse. Track transponders and axle counters know which trainset passed a specific piece of track, but the trains don't know which track transponders link together on the same track. They don't even know which track they'll be going down next because that's up to the signallers controlling the points, not them.
2. Planes fly in the air and can send omnidirectional radio signals with little to no interference. Trains run on the ground, often in valleys and tunnels. There is no guarantee that trains broadcasting "I am here" signals can be picked up by the next train within the stopping distance.
Many trains did broadcast "I am here" signals, but once speeds got faster, the broadcast range (about 1km) wasn't far enough for a train to stop in time before a collision, which is why trains need to communicate, using on-track wires that run the length of the track, to each other and central control. Which therefore means that the central control system needs to work, and when it doesn't, the trains have to stop.
Planes use radio signals because they don't have signalling cable following their entire route. Trains do, and it's usually much more reliable than radio.
They can upgrade the system so that the minimum distances are smaller. Sure. Except, such hypothetical upgraded system was already in place, since 90s, and __that was what went down__.
If you were to then ask me, okay, why did they get the trains apart before committing with the downgrade into the older system. And that would be the right question. It's such a braindead move to not do that before executing the full downgrade. It's not like trains were moving when downgrade took place and it couldn't be done, the newer system going down triggered emergency stop of all trains. Train systems are intentionally built that way. So they could (load-bearing-)just ordered the trains to get spread out so there will be enough distances between trains so the older version of "train TCAS" would work correctly. The decision to NOT do that, with hindsight, is downright reckless.
By the way, if all reports on this matter is to be believed, the required distance of the older version of the "train TCAS" in operation was in excess of 25km, or 15 miles. That's long. It can be shortened. Sure. But then again, there was such a newer system already in place since 90s that has much less minimum distances than that, which went down.