it might not even need regular rails. there are trains that work with rubber wheels (paris subway had that for decades already), so it would really just be fancy busses with guide-rails.

EDIT: pretty much the same idea is mentioned by several others in this discussion, so it seems not to be to far fetched.

of course, given the popularity of public transportation in LA, it makes sense that they are going for the ability for people to use one vehicle from their home all the way to their destination. i just hope they focus on actually getting the most use out of the tunnels, even if their initial vision doesn't work out the way they intend.