Here are some Internet sources for train songs:

Choo choo. - a list of 50 songs with lyrics and discography.

RailServe - Train Songs and Sound FX - a list of CD's with links to CDnow, which sometimes provides track listings and samples.

Wes Modes' Train Songs - a list of about 500 songs and performers.

Philip Pacey's Music and Railways - a truly amazing list of mainly classical, but some pop, music inspired by railways.

Chris Coleman's Railroad Songs - a list of about 1000 songs and performers, some with dates.