True self-drive would be much better:
Which is what the Google cars are.
From what I've seen, Google is so far ahead of the pack in the self-driving car technology they're going to take over the roads first. They already have permission to be driven in CA and NV.
I'd be curious to see them do some serious rural driving on dirt roads and such, that's where it gets fun. I suspect Google cars probably can't go anywhere Google maps can't go which is most likely the limiting factor but we'll never know until someone lets us get behind the wheel and not drive
one of those cars.
The train thing is a pretty simple thing to do comparatively speaking with the technology they already have that makes the cars sense the distance of advance vehicles to auto-break and so forth. From there making them follow at an even pace isn't so hard really. All the smarts has to be in the lead car at that point, whether human or computer, the followers just need to be able to match the direction and speed of the next car and pray the lead car stops in time!
All it would take is one crazy driver to jump in front of the lead car, leaving it insufficient braking distance, and put a hard slam on the brakes to start a nice chain reaction that would be quite ugly.