A year or three ago I did some work trying to put together a noontime 1 hour FM radio program for the Albuquerque classical music station. I never completed it, but in the process I collected over a dozen CD's of railroad related music. Of course there was significant duplication, but I was amazed at the volume of material that's available.
Among my favorites is Steve Goodman's recording of "The City of New Orleans" - significantly different than Arlo Guthrie's. Goodman was the original composer, and I like his version better.
Also, there is quite a bit of stuff in the strictly classical realm. Like Johann Strauss' "Pacfic 231", and Villa Lobos "Little Train of the Andes". Also, Honeger (?) composed a piece to commemorate the opening of the Chunnel. The more you dig, the more you find. But that shouldn't be too surprising since the railways were the major form of transportation for 100 years.
Happy hunting.