When was HEP introduced...

As someone with more than a passing interest in PRR, I have to kick myself for forgetting the “Keystone”!

I like the definition that says “Headend Electric Power”. That seems clearest (though seldom used by anyone, including your average railroad crews).

Metro North’s FL-9s used some really smoky diesel engines for the HEP that blew a rich black cloud out the side of the loco when they were fired up just before the trip started in Grand Central. Those were the days they ran diesel under Park Avenue, and even for some years after.

Their MK rebuilt FL-9s (I forget the designation) used the prime mover to generate HEP so they had to sit and roar at all the stops along the way.

Now they have all been retired from passenger duties, replaced by the Genesis locos with third rail power. At least under third rail they seem to do a better job keeping the lights and AC going across the millions of switch points in the Terminal than did the FL-9s under third rail power.