Flashback (1990) was one of a number of movies filmed on the standard gauge Rio Grande system. You can find more information about the movie itself in its entry in IMDB. (Personally, I haven't seen it...)

Equipment

D&RGW GP9s 5941 and 5942 were relettered "Pacific Rim R.R." for the movie, along with the Ski Train equipment and caboose 01465.

Filming Locations

Filming locations were primarily around Glenwood Springs, although initial scenes were at Denver Union Station and around Arvada, and the final scenes were on La Veta Pass. (From Bruce Collins via the D&RGW list, 2002)

All of the train interior scenes were filmed in full-sized wooden mockups in an Aurora, CO, warehouse in 1989. (From Dave Schaaf, via the D&RGW list, who worked on the movie.)