Besides, trains run on tracks, each of which accomodates one train at any given time and rarely intersect; additionally, there's all sorts of signalling and saf

Besides, trains run on tracks, each of which accomodates one train at any given time and rarely intersect; additionally, there's all sorts of signalling and safety infrastructure embedded with the track... In stark contrast with lane-swerving cars, intersections everywhere, temporary rerouting because of roadworks &c. Running trains must be orders of magnitude simpler than fully autonomous vehicles. As for planes, they have a massive benefit in being engineered and maintained for levels of reliability no car can ever hope to achieve, having an awful lot of empty air around them (not to mention being able to move in the Z plane, too, to avoid collisions. I don't doubt there are many lessons to be learned by designers of autonomous vehicles from work already put down in the fields of trains and planes - however, I'd argue they are very different problems.