Dave, I became a Pennsy fan as a child the day my father brought home an American Flyer freight train powered by a cast metal K-4 Pacific. He ordered the trainset following his return from the war, but it took nearly a year before the A.C. Gilbert Company could begin using metal for their trains. (There was a metal shortage during the war.) The trainset was in a 'transitional' period where the loco and cars were 'S' scale, but the wheel sets were designed to straddle 3-Rail Lionel style track. (Later, that loco received an 'S' guage wheel set to allow it to run on two-rail 'S' guage track, but by then the loco was cast in plastic.) I loved my K-4, slept with it, and eventually wore all the paint off of it! Today, I have a few Pennsy HO locos but the majority of my collection is made up of ATSF, SP, UP and my favorite: the Pacific Electric.
-Ed