"Mixed Train Daily", the Beebe book cited above by mwb, has long been one of my
"bibles". Beebe and Clegg found these railroad operations so interesting, they
traveled the country recording them right after WWII, for which we can give thanks. My mother used to ride from Shelbyville, Ky. out on L&N's Bloomfield branch
to visit her aunt on a mixed train comprised of assorted freights and a "Jim Crow"
(center baggage door) combine. At the end of this train's operation it was pulled
by a diesel switcher, I think a Baldwin. An old issue of MR has an article with photos
on doing a layout around a Wisconsin line running mixed trains, powered by diesels.
With the thread running on mixing unmatched passenger cars, and mixed trains, you
can get an idea of what runs on these rails here.