MMDer John Preble sent this tip and link about a toy train that enables
kids to create their own tunes.  The concept was created by Brazilian
designer Ricardo Seola in 2009.  The toy train is pictured at time 1:42
in his concept video at http://www.youtube.com/watch?v=lhDT0nwjZQM
 

Seola's idea is currently under discussion at Quirky, a forum for
inventors, which investigated different ways of producing the sound
mechanically including the tuned metal comb from a real musical box
movement.  See the discussion summary at
https://www.quirky.com/products/526-Sound-Track-Musical-Train-Tracks 

Robbie Rhodes - MMDigest