The C&TS was very much in the running for the railroad scenes for BTTF3. The producers and location managers were the same crew that had just finished Indiana Jones #3 in Chama and thought the C&TS would be perfect for BTTF3 also.
Alas, delays in production required them to be in Chama in January, not very attractive to Hollywood types (too cold), so they went to "plan B" on the Sierra RR. The real shame is the production was further delayed and they shot on the Sierra the next summer, plowing some $2-3 million into the local economy.
Sierra is a magic place. I managed to see it back in the late 70's. Riding a doubleheader behind 28 & 34 up Dry Creek Canyon in the dark was totally cool.
BTW Sierra 34 qualifies as a "semi narrow gauge engine" It was built a month after the 480's, has a lot of small detail parts used on the the 480's, it even looks like a stripped down, widened out K36.