Actually, there are plenty of autonomous trains: https://en.wikipedia.org/wiki/List_of_automated_train_system...

The problem is automatizing the existing infrastructure, 'cause it requires great effort (so, money and inconveniences).

I don't think however that the lessons there are applicable to cars as well, and while I think we could and should invest a lot on train systems, the final goal for cars should be reducing them, not replacing them with autonomous systems.