"Impractical"? Nonsense.
Whether HEP dies more or less often than the loco itself is pointless; result is the same in every case-- no HEP. Bring in a spare freight loco from UP, CSX or wherever, your now-moving-again train still has no HEP. And lots of unhappy people.
On the once-a-day long hauls, a HEP failure immediately becomes catastrophic. Check out the latest TWA posting for the latest horror story. People understand things can break, but expect them to be fixed. Fix it and you're a hero for an hour, but if you fail to, you're an idiot forever. Human nature.
A cheap, simple Cummings diesel in a cheap car (anything that can move) and a used 480V 3p alternator is all it takes. It doesn't even have to be a full 65/75 Kw setup-- just enough to maintain a relatively decent amount of comfort. Not like they'll get used every day (or week). Just something cheap enough that can be left at a strategic location without someone else wanting it elsewhere (like they would a NPCU).
As far as stress on a P42, the only place on the system where demands can exceed what working locos can provide is Raton Pass. Lowest Continental Divide crossing, but it's all 4% plus. Here, Amtrak could have taken a cue from my local power company; Mondays, when demand is highest (Laundry day is still traditional here, believe it or not), once that last generator hits 100%, we flip a switch and all the homes in the area start cycling their water heaters off and on.
And what makes putting the same stuff inside a NPCU better? Same function. P42's WAY outpower what's needed on Midwest trains, so there's no "stress" there. Even the old F40 was overdoing it. Sure, they should carry spare HEP, for the exact same concept, but them and not something system-wide? The long hauls don't use cabbage cars.
An advantage of a HEP car is that a loco with dead HEP can still pull revenue while on its way to the shop, this freeing up a good loco.
The debacle reported in the latest TWA could have been averted, at least partially, with a HEP car: Loco in front could have powered "what it could", and a HEP car in the back could have powered the rest.
And also, the mess with the Florida train with the melted HEP connector could have been resolved quite easily; Loco powered the train from cab on back to the bad car; HEP car (in the rear) could power the rest of the train (behind the dead car).
Finally, I've been aware of Amtrak's budget problems since Amfleet cars were still being delivered.