Hey Robert, in reading your post I am making some assumptions. I assume you are running the engine by itself with no cars couple to it. If there are cars coupled to it it may be that one of the cars is creating the short.
If the engine is running alone a couple things you might want to check are, if the draw bar connector for the tender is bent down somewhat or the screw holding it is loose and it may be swinging over and hitting the center rail creating a short circuit as the train goes around the curve. Also, check the cow catcher as I have bought some used engines and found the cowcatcher either damaged or the screws holding it loose and it is dropping down and hitting the center rail.
It would help if you could give us the number on the engine and possibly a picture of the bottom of the engine. Good luck on solving your short circuit problem.
Jim Lawson