Amfleet,
Here's a couple of other things about HEP. Years ago there was no electricity from the engine. If you recall some older discussions here on AU, trains used to use steam to heat the coaches. The oldest trains never had any electric in the cars at all. As we approached more modern times, then each car would have a small generator to provide electricity.
Some of the generators ran off of the steam provided by the engine. In other cases the generators were attached by gears to the cars axle, so when the train was moving you had electric power, however once it stopped off went the lights.
Now back to modern times. Each diesel engine built for passenger service in addition to the main engine, has a HPU (
Head-end
Power
Unit), which is really a separate smaller diesel engine with it's own generator attached. The HPU is what provides the 480 volts AC HEP to the train.
There are two major reasons for the HPU; one is that you are not stealing power from the prime mover, as the main diesel engine is called. Second if the prime mover fails, depending on the circumstances of that failure, you may still be able to provide power to the coaches even if you can’t move the train.
If your train has only one engine then obviously that engine is not only moving the train, but it is also providing the HEP. However, if your train has 2 or more engines, only one engine is providing HEP. The other engines only provide additional motive power to make the train move, they don't combine with the first engine to provide the 480 volts of HEP.
The reason for this, without getting highly technical, is something called phasing. When you use AC power, or what’s called
Alternating
Current, essentially the power switches from negative to positive 60 times per second. It’s really more complicated than that, but for our purposes here that explanation will do. If you have more than one generator running, then it must be precisely in sync with the first generator, or properly phased as it’s called.
If it’s not, then you have big problems as one generator is sending out a positive current, on any given split second, while the other is putting out negative power. Getting two HPU’s to properly phase, is far too complicated for the limited benefit that it would provide on a train. Getting three HPU’s in phase is even tougher. So if you are on a train with three engines, only one engine’s HPU is actually running. The other two HPU’s are turned off.
If you want to better understand AC power and phases, then you can check out
this page from How Stuff Works.
Normally to my knowledge, Joe may know better, usually it's the lead engine that provides the HEP. I think that it's done this way, as it's easier for the engineer to monitor the HEP and turn it on and off as needed, like when adding road railers and MHC's. However if necessary, any engine can provide HEP.
Once coming back from Florida, on the now defunct Palm, I saw this happen. We were coming through the Carolina's, when I thought that I saw some unusual black smoke coming from one of the two engines as we rounded a curve.
A minute later we lost all power on the train. Thankfully my room was not on the side of the train where the sun was, as it started to get rather warm even on my side. We ran without power for close to an hour, until we reached our next stop. Then the engineer had to walk back to the second engine to flip on it's HEP, at least that's what I'm assuming since a minute after he climbed into that second engine we had power again.
I don't know precisely what went wrong in the lead unit, but the OBSC did tell me that there were problems with that engine. I’m assuming that either the HPU generator or diesel failed, and created the smoke that I saw coming out of the lead unit. Thankfully they didn't need to cut that engine off the train and delay us.
we've missed you and your train expertise around here lately. Hopefully you've solved all your computer problems and are back for good now. 
