> Detecting objects (and future objects!) on a train track is still a hard problem.

In this case the “object” was another train. That’s not the “hard” part of the problem. This sounds like an apologist comment.

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.