Not sure what you are planning to do, but a few suggestions!

Folk (-ish)
Cannily Cannily (McColl)
Song of the Iron Road (McColl)
Travelling by Steam
Train Song (Pentangle)

Pop
Chattanooga Cho Cho
Locomotion?
Homeward Bound
Last Train to Clarkesville
Midnight Train to Georgia

Blues
Midnight Train
Midnight Special
Freight Train
It takes a lot to laugh (It takes a train to cry) Dylan
Life is just a slow train crawling up a hill (John Mayall)

Country & American
Orange Blossom Special
Wreck of the old 97
Casey Jones
Last Train to San Fernando
Rock Island Line
City of New Orleans