If you remove the block signaling system and use in-cab signaling, you still need to know where the back of the train is. Otherwise a train could run into the back of another in the fog or heavy rainstorm. Currently, railroads use FREDs (flashing rear end device) on the last car of the train. Some FREDs are more intelligent than others but would benefit from being able to send out their location via GPS.
Longest trains are under five miles so you could use that or whatever the limit for that line is as a safe baseline. Unless you're worried about trains separating.
That's not an actual problem. You simply have 2 trains each broadcasts it's location. It's unexpected separations that might be a problem, but those are currently very rare and dealt with seperatly than static signals.
It's like you need to design a nuclear reactor to deal with an aircraft hitting it. You don't need to automate the response becase that's a rare situation.
Well, I read about one instance where it happened, and the brakes didn't apply. (Not sure how that could happen other than a mess-up in building the train. You'd just about have to have both angle cocks closed.) Both fragments continued, for miles.
Can't cite a source (I think I read about it on the Altamont Press discussion board). And that's only one instance. And it could have been reported incorrectly. But it appears to be possible for this to happen.
https://en.wikipedia.org/wiki/End-of-train_device