Charles Bronson is on a train with an all-star cast in Breakheart Pass (1975).
People are dying, but no one knows why, or who is doing it. Can Bronson figure it out after he finishes fighting with Archie Moore on the roof of that train? (And BTW, that fight scene was filmed atop a real moving train. No CGI there.)
Also in this movie: Richard Crenna, David Huddleston, Charles Durning, Ed Lauter (a pal of Bronson, they starred in several movies together), Jill Ireland, Robert Tessier, and Ben Johnson.
And if you like steam trains, you really need to see this one. Truly a great movie, made back when movies were worth seeing.