right, it might not work for a regular sized train, but if it can fit a car, it can fit rails and a custom designed rail carriage with the same advantages of a regular train.
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.
IIRC the main heat source is the breaking of the trains combined with the high frequency of them. I suspect that using many cars will still release a lot of heat into the tunnels.
The trains in the narrow tunnels push air around, so recent stuff like full height platform screen doors help improve conditions (allowing platforms to be air conditioned without having to air condition the tunnels)
