I think the best instrument for a subway is a saxophone - it seems to be a perfect match for the resonance of a subway tunnel. You'll always find a sax or two in the long tunnels that lead to the subway at O'Hare in Chicago. If my layover is long enough, I head for the subway so I can listen to the music. I keep hoping I'll meet Candy Dulfer down there some day. So far, I've only come across male sax players in the subways.
-Joe Offer-