Those are usually "closed" systems, that don't have to deal with unpredictable events that often.

And in non closed systems the set of problems that prevent fully autonomous cars is the same set of problems that prevens fully autonomous trains, ie, recognizing shit getting into the lane of travel but not panic braking for random trash.

If there is something on the track the only option is hitting it. You cannot switch track (even if there is a switch on the tracks, it can't safely operate in time) You can hit the brakes, but your braking distance is so long that you won't be able to slow down much before you hit it.