May I point out that there's still a big difference between "in operable condition" and "in service."

I suspect that most, if not all, of the ones we have listed as "in operable condition" only "come out to play" for either special events or when nothing else is available. The couple I listed initially are (as far as I know) the only ones you are LIKELY to see in operation on a random visit.

There is an advantage to their use to some extent, because they CAN be used on one engine (1125 hp or so) for a short, low-speed train, and engines can be alternated to reduce overall running time. But you're still dealing with wearing out 36" wheels. (That trick is also used in British preservation with double-engined "Westerns" and "Deltics" in guest service at various excursion lines.)

From what I have seen 6900 runs fairly regularly at Spencer. I think the F3A is more of a "Special Occasion" unit but does see significant use; especially when they have two different trains going

Lorain & West Virginia 101 is the only operable motive power for the railroad so while they don't run every weekend if a train is running it's being powered by the E8.

Southern California (formerly Orange Empire) Railway Museum at Perris, CA has UP 942.
Built in 1953. Sold 1972 to C&NW #510 for commuter servicess; then to RTA/Metra in 1977; sold 1989.
To Orange Empire Railway Museum in 1997. repainted as UP 942 2011-2012.

For more information for years 1989 to 1997 see Utah Rails https://utahrails.net/up/up-diesel-index.php