What keeps the train on track?

Another piece of stories by Richard Feynman ( 1st , 2nd and 3rd parts). This time, the physicist talks about what questions they liked to ask in the scientific community of MIT , in order to return an arrogant student to the earth. Questions that sound very simple, but the answer is not always so obvious. The first question was about the reflection in the mirror (this topic has already been raised on Habré), and the second question is more interesting: what keeps the train on the tracks?

Original clip .