What difference does that make, genuinely?

Are there systems that can warn drivers and prevent collisions without external signaling, like ACAS?

Yeah, that's a solved problem. Did you even read my whole comment? I answered why "just use the system from self-driving cars" wouldn't work and in fact would be less safe than the current system.

What's the stopping distance of a high-speed train at full speed? I don't know, but I bet it's kilometers, not meters.

Detecting an object on the tracks 2 km away (say), as opposed to beside the track at the same distance, is in fact a hard problem. The tech used in self-driving cars is not going to solve it.