Once you tell the automated system that you are leaving the train, you would take control of the car and change lanes. The car behind you would sync up with the car in front.
Same thing with joining a train in the middle. You tell the system you want to join, it talks to the cars to the side of you, they provide space, and you merge.
Right now cars have radar and laser systems for measuring distance to the car ahead and providing better cruise control, but you still have to steer, and you don't know what the other car is going to do until they start doing it. In the car train all the computers would be chatting, and every car in the line would know the instant any driver touched their brake pedal or the steering wheel, and they would allow space for independent movement.
This kind of system, however, would require 360 degree active collision avoidance. If, at any time, something happened where communications were broken or jammed, or vehicles not part of the train appeared to be on a collision course with the train, the people in the cars could not react quickly enough to avoid collisions. However, with 360 degree radar and laser object detection, the car itself can attempt to avoid as much of the crash as possible.
