> There's no physical connection with the piston in the pipe, just a big magnetic field. Without air pressure differential to make an air-bearing the friction between the magnet and the pipe wall might make it impossible to drag the train along. In a way, having a built in emergency brake might not be that much of a bug...

I'm not sure I follow. There has to be some way to start moving from a dead stop, so the plug will probably already have some kind of wheels or bearings to allow it to get moving until there's enough velocity to make the air-bearing work.