My first check would be the bulb, one failure mode of bulbs is to go short circuit, although this is rare but possible.

A second possibility, I beleive these are basically Trix design coaches, there is a trick when taking them apart to make sure things are put back correctly, if the body is put on the chassis the wrong way around you can get a short. This is related to them being a 2 rail design adapted to 3 rail.