the Boston Blue line switches from overhead power to third rai,l when it is at grade level it is overhead when on exclusive right of way it is third rail, when switching the other is deactivated so is safe.

the cars on the Blue line are "Light Metro" very similar in size to the HART cars.  
it is not difficult to put an operator station in the HART cars they already will have controls for use in the event of emergencies on in the shop area.

the cars could even run automatically on the elevated section and an operator get on board when it comes to ground level.    

there should be no changing of vehicles this causes much confusion and loss of ridership