Intact 3/16ths O gauge Flyer trains are intact and out there. I started picking some up maybe 20 years ago (after I'd rebuilt a fleet of prewar Flyer that had been donated to a museum). Flyer 3/16ths is prone to zinc pest (which I've dubbed Dorfan's Disease in recognition of Dorfan's pioneering use of impure zinc when diecasting trains). You're not likely to cross paths with an intact 574 NKP 0-8-0; legend has it there's an intact specimen from frame to drivers, but normally the frame is junk and the drivers crumbled/crumbling, theri shells often fall apart when screws are backed out - if you find one, it's pretty much a lock that it will have repro wheels which unfortunately are incapable of being allowing the engine to run as they're made from a pretty soft metal. UP 4-8-4s tenders frequently suffer from shell rot, frames bowing or splitting apart, truck sideframes are prone to disintegrate, lead and trailing truck wheels on all the engines are prone to rot, diecast freight and pasenger cars warp/crumble - you get the idea.
That said, aside from the elusive 574 NKP switcher, it's possible to find intact/running examples of the rest of the diecast prewar Flyer 3/16ths O gauge engines and cars. At this point, if it's intact, it's most likely going to stay that way. Parts are available to repair engines (a lot of the parts showed up in the engines after the chassis/mechanisms were modified for 2 rail S gauge operation after WW 2). Some parts like linkage have doglegs bent into them so they can extend around the wider O gauge wheelbase. You'll pay a premium to get an intact UP Challenger (4-8-4), but the rest of the engines are obtainable for not too exorbitant prices.
Running those prewar Flyer engines on 3 rail track always gets a look of "how'd you do that?" when folks don't know that Gilbert made 3 rail trains that were built to S gauge dimensions. They look great pulling Marx 3/16ths O gauge tin lithographed cars (which look much nicer than the stamped steel cars Gilbert produced). Flyer's diecast freight and passenger cars look very nice, but they weigh a lot and make it tough to pull a very long train; only a couple prewar 3/16ths scaled engines showed up with a traction tire. If you're patient and willing to spend some coin a nice prewar Flyer collection can be put together, and they can be made to run dependably
