Self driving trains will require more coordination, with central dispatch to tell them when/where to go. That leaves the intelligence on the train much more basic.
They have signals to tell them “stop”, they have speed limits posted, they have route knowledge. All detectable by computers, even without changes. Fast trains have in cab signalling in any case.
They have no decision about where to go (signal box sets the points/switch track), they have no decision on when to go (signal goes green)
The only bit which may need human input is the “ok to depart” notification when all the doors are clear at a station.
There are hard limitations on sensing onboard the train. Tracks that bend around hills, etc. I have some exposure to wireless communication for trains, and one of the limitations is that you can often not get a line of site even from one end of the train to another.
Since railroad tracks rarely get up and go walkabout, you can instrument them (conduction, video, lidar/radar) to the limits of your preferences and/or budget. Information can be relayed to both central control and individual trainsets.