GPS is actually a very small part of any location verification needed for a PTC system. It is the communications, and embedded locational confirmation, from things like DSRC messaging, from wayside devices, and internal programming within the system that would establish, verify (from multiple sources) and confirm (based on all locational relevant data being in synch from all sources) for things like track location, track speed, signal aspect, and the like.

It's a mostly a closed loop system for crying out loud, it's a railroad, with what is for modern communications and computing capacity, only a very limited set of parameters to incorporate, integrate, and communicate with.

We're doing this with random open systems, complete with multiple pathways for millions of moving automobiles and trucks, confirming many more inputs, and doing it very well. Now, if we can only figure out a way to keep the hairless chimp brained vehicles behaving "rationally" or predict their likelihood of stupid, we could roll out the google car tomorrow.

PTC has been more costly and taken longer, not because of any lack of technological capability, but from the decision, that instead of building a system with well defined user needs, on a clean architecture, that the architecture would need to include full integration with every current vendors existing systems. That is taking a lot more code, a lot more communications, a lot more integration, and thus, a lot more money, and a lot more time..... And, let's not kid ourselves, doing it that way, was more about the intersection of vested interests with politics, than rail safety.

At the end of this, is the fully automated railroad, to compete with the fully automated driverless truck.....which is already here. And, it will reduce the likelihood of these events by another 90%, kind of like establishing standard time, and the book of rules, did. Ultimately, the human, in the term "human error", is coming out of the loop.

These are awful. I've been to a few too many of these in my career, and it always sucks.... My deepest condolences to the families and coworkers of these folks, who are in shock and grief right now, and in need of support and prayers from all.