the HEP is like an alternator in a car....charges the batteries in the control car. When the HEP goes down, especially inbound trains on long distance trains(30+ miles) the problem becomes pretty real. The batteries in the control car can't be charged when the HEP goes down. The batteries are left to their own juice, which is very little seeing as most control cars floating around have been circulating for 15+ years and even the new 1800"s can't be trusted for a train to "go in dark." The batteries charge the radio, headlights, all electronics etc. The engineer can't control the train from this position, so they must go to the locomotive and "go in reverse" and let the conductor talk him back. The go to answer in these situations is to have the engineer pipe the train in, as mentioned a few sentences ago, with the Conductor leading the move...unless there is a close train behind for a double draft or a rescue unit to cap ontop of the troubled set