Google Engineer Alexander Chen made music with subways in Conductor (2011). HTML5 and Massimo Vignelli's subway template were fused together in this beautiful piece of interaction design, with different variables controlling pitch and length of note, all shaped by the shifting trains and their respected trajectories.
Here is a description by the artist himself: "Conductor turns the New York subway system into an interactive string instrument. Using the MTA's actual subway schedule, the piece begins in realtime by spawning trains which departed in the last minute, then continues accelerating through a 24 hour loop. The visuals are based on Massimo Vignelli's 1972 diagram."