The Hugh Williams song "Travelling By Steam" (also covered by Fairport) is a good one which I play a fair bit at sessions and open mics.

Another good one is Bob Kenward's "Old Country Train" about the Hawkhurst Line in Kent...I played it for a railway enthusiast with a special interest in this line, and he was amazed at how much research must have gone into the song to get the terminology and "feel" just right.

From the USA, Eels' "Railroad Man" is both a nostalgic look at at bygone railroad days, and a metaphor for someone feeling disconnected from the times he's living in.