Thanks to Jason Fellows for hipping me to this.
Conductor (2011) by Alexander Chen. Video capture. View live at: mta.me
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.
More details at: