Singing in a train is as good as singing in the rain..
Another one or two about romances that had one of the characters in a train, are these-
Jiya o, jiya o jiya kuch bol do..Rafi singing for Dev Anand.
The same idea, later, with Kishore Kumar for Rajesh Khanna singing to Sharmila on the darjeeling mountain railway.
Mere sapnon ki rani kab aayegi tu..I grew up watching this, and it's still a favourite.
Another old gem with Hemant Kumar for Dev Anand singing to Waheeda Rehman.
Hai apna dil to awara na jane kis pe aayega from Solva Saal.