Has there ever been a case where two self driving cars collide with each other? I hate to be part of the peanut gallery, but as a totally naive person when it comes to railway safety research it really does seem amazing train collisions can’t be easily prevented with gps, lasers, sonar or various other trivial ways. Even the crash in question seems like it might have been easily prevented by having a second human in the loop. Maybe I and others just suspect the difficulty isn’t so much technological as it is financial.

> Has there ever been a case where two self driving cars collide with each other?

I don't think that's relevant to trains. Cars can avoid a collision much more easily than trains can. Cars can swerve, and if they are at a distance at which line of sight tells them they will hit an obstacle, they are moving slow enough to stop before hitting that obstacle. Trains have to stay on tracks, and they need up to 4 kilometers to stop moving.

The constraints are very different, so the solutions and tradeoffs will be different too.

As far as I am aware, there has been no non-supervised train in a general train network. (They exist in closed systems.) However whether the train is self-driving is really not the issue here. In current railway safety systems (even from the 80s) a crash is about the same probability as random bitflips. However reality is too messy, so that the availability is not satisfactory enough to not use a human fallback, and then you are back to the assisted human error rate.

Current operation of a train is basically self-driving and supervising driver. So if you suggest "just use self-driving", you make the system less safe, not more, because it means just getting rid of the human supervisor. This is not currently done, because it would be too insecure. It might be done in the future, but rather because less safety is going to be accepted in favor for getting rid of the costly human.

A self-driving train will happily cause a crash, if the target is "drive with high-speed in that segment currently occupied by another train". The issue is that that becomes a target, not that there is a human in the loop.