There's an instrument for all to use inside the station at 34th Street Herald Square. The audio installation by Christopher Janney is called "Reach New York: An Urban Musical Instrument" and was created by Christopher Janney (who you may remember from those musical stairs in Boston) back in 1996.
Perhaps because so few people know about it (we didn't), Transportation Nation recently brought attention back to the inconspicuous green box suspended above the uptown and downtown platforms of the N/R train. A plaque near the instrument explains:
As you reach up and wave your hand in front of one of the eight "eyes" in the horizontal green bar you will interrupt a beam of light. This will activate REACH, creating sounds from melodic instruments (marimba, flute) to environmental "sound images" (Everglades, rain forest). Because the piece is on both the uptown and downtown platform, you can interact with others across the tracks.
Take a look below or head on up to 34th Street and give it a try.