HEP = Head End Power; it's a concept which predates 1900 (GE made a DC system which used a power car in similar fashion to the one in this discussion). In it's most recent incarnation, it started with Amtrak, who wanted to get away from all the 32, 64, 110 volt electrical systems, steam heat, and Waukesha, ice, mechanical, steam ejector, and electro-mechanical air conditioning systems found on all the cars they inherited from the various freight rrs. It was a case of standardization and cutting down on inventory in the supply warehouses, not to mention the mechanical forces having to learn the complexities and quirks of each individual system.

The UP may call it something else (they usually do this sort of thing just to be different, like "FEF" for the 4-8-4 steam engines just so they wouldn't have to use "Nothern Pacific", at least that's the story I read a while back -- they're doing the same thing with terms the BNSF uses today). As I understand it, the fleet of "toys", as the passenger cars are called in house, have been heavily modified in many ways since their days in passenger service (ABD has replaced 22, 24, and 26-C; IOW no more graduated release), and no doubt the electrical systems have been heavily modified too. In order to be hauled in an Amtrak train, they must be Amtrak HEP compatible (480 v, 3 ph, two sets of plugs on both ends of the car, with the pins set up for a certain specific phase rotation). Canada (specifically GO Transit) uses 508 three phase. I believe the only SP business car not so modified was the Sunset -- who knows what has happened since it turned yellow.

Besides, when I was in the Navy, it was called "Shore Power".