After the advent of Head End Power (HEP), which is trainlined 480 Volts AC 3 phase and powered from the auxillary alternator or stand-alone diesel generator set in the rear of the locomotive, Amtrak removed steam heat and axle-driven generator sets from their fleet as time and money permitted. For several years, up until roughly the mid to late 1980's PVs could ride the rear of most Amtrak trains and power up their cars with a single plug/outlet tap on one corner of the PV, at either end. As you might surmise, this situation lent itself to restrictions in car placement, but allowed the PV a mostly reliable source of power and virtually all cars doing this were able to eliminate troublesome axle-drive sets by installing 480VAC to 32/36/110VDC transformers as the case may be.

Eventually, Amtrak mandated that all cars will be wired for complete pass-through HEP, which also must meet Amtrak installation and inspection standards.

As an aside, this mandate caused many PVs to cool their wheels as the financial undertaking to comply with this was and is still quite substantial. If I'm not mistaken, part of the HEP requirement was that steam conduit be removed in its entirety (including the cost of asbestos abatement)- which was a matter of course in most cases on account the room required to pass electrical conduit through the body bolster necessetated the removal of all unused conduits (steam, for one).

I've also heard them refered to as quarter-taps, as in just one of four corners having a connection.