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...

Another difference between humans and bulk cargo WRT transport systems is humans will self unload and self load into slower and less ecological and less efficient more expensive transport when and if necessary. Surely in the event of a breakdown a fleet of smokey diesel charter buses and taxis would appear to transport the people. On the other hand, the railroad has a real problem if a thousand tons of coal cars breaks down in the middle of a route, that coal isn't going to walk itself into an adjacent pickup truck or something.

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.

You'd only need to move the disabled carriage / section to one of those to enable at least some capacity for the line to continue operating.

That was my interpretation of the comment.

I believe "siding" is the word you are looking for.