They can switch from permanent magnets in the middle to electromagnets near the stations, or use linear motors (similar thing, just not mixing it with the stuff that keeps the tube afloat). If they do it right, braking will generate 60% to 80% of the power that was used to launch the tube, so sending it off again won't cost as much.
The problem being that to maintain 1G or less acceleration, and achieve high speed in the middle the motors have to be mounted on the train, and the stator mounted on the track for the entire length of the track. You wouldn't ever be coasting, it would always be under motor control - or at least it would be available even if you apply no power.
If it's a linear induction motor, then the stator can be simple aluminum rail that slots into the motor. The motor would be mounted on the tube, and the stator would be one or more rails as part of the track/evacuated tube.
