All of the current cars are self powered - no HEP ... no connections between cars except MU/door controls. If the pan goes out on a trailer the trailer has no power (except emergency lights)?
Most EMUs built after 1945 or so (think the first CTA 5000s and later) run off of two electrical circuits - the traction circuit, which is the 600v/1500v/etc circuit which powers the motors, air compressor, and any other "heavy duty" components; and the low-voltage circuit, which is where things like the lights, air conditioning, and (perhaps most importantly) the motor controls get their power. The traction circuit isn't trainlined unless the cars have a special jumper built in. All CA&E interurban cars were all equipped with such jumpers, and A few of IRM's PCC "L" cars have such jumpers so we can run a motor car without poles in tandem with a car with poles (CTA 22 & 41 are run this way).
The low voltage circuit, on the other hand, is always trainlined through the many pins in the MU-type couplers. This way, you can run a trailer car which can accomodate passengers just like any other motor car. Its motors will be cut out, as will its air compressor (if so equipped) but electricity from the low-voltage circuit supplied from other cars through the MU coupler will keep the lights, HVAC, and motor controls running. The latter is extremely important because if a pan goes out on a lead car and it can no longer operate as a motor, its cab can still control the other cars via MU, hence it is now a "control trailer."