Look at the electro coupler mounting plate.
It often comes loose from the truck frame. If you can wiggle it you have a problem and it can short both halves of the truck frame together if it gets loose enough.
See link for repair info:
http://www.tuveson.com/bigboy/..._big_boy_coupler.htm
If that is not it unplug the railsounds boards and see if that fixes the short.
Carl