First thing is to find out if it really is short-circuited, or has it gone open circuit. Put it on the same track with another, good, engine - if the good one moves, then it has gone open circuit - if the good one doesn't move, then it's short-circuited. Open circuiting may just be caused by dirty wheel contacts.
Short circuiting can often be caused by things, usually track pins, being picked up off the track.