The mystery of the magnetic train

The physics behind the magnetic train that I linked to a little while back, as well as homopolar motors (which I did a long time ago)

[A]ccording to my experiments, it is necessary to make sure that the two magnets on either end of the battery have their North poles pointing in opposite directions! Otherwise, they are either both pushing or both pulling, and the “train” doesn’t move.

That was something I had thought would be true when I saw the video.