2.31. A train pulls into a station, but none of the waiting passengers move. (MN)
2.31 answer: It's a model train set.
2.31a. Variant: The Orient Express is derailed and a kitten plays nearby.
2.31a answer: The Orient Express is a model train which has been left running unattended. The kitten has playfully derailed it. (from Bernd Wechner)