The oldest telling of this puzzle I know of is Wigner's account of Max Born posing the question to von Neumann in the 1920s: <a href="https://www.youtube.com/watch?v=VTS9O0CoVng&feature=youtu.be&t=1006" rel="nofollow">https://www.youtube.com/watch?v=VTS9O0CoVng&feature=youtu.be...</a> . In that version, it was a pair of bicycles, not a pair of trains.