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I removed the rear truck, tried nail polish (clear top-coat, to be precise ), it still shorts. I run it slow so when it shorts I get a snap shot of exactly how it's positioned and can't see any metal-on-metal. I only saw a spark once but I'll keep trying and looking for flash spots...and gremlins.
Edit - just saw a spark above and between the last two drive wheels, from inside the gear box through a space in the frame. Could this be a bare wire inside?
it looks possible the pilot wheels could touch the cylinders or any parts nearby on curves or wobbling around. With the engine upside down lay a piece of snap track on the wheels, slowly tweak it around and observe the wheels.
Loose wires, or wires that have got caught in-between things when the shell has been assembled to the frame so they get nipped or rub against the flywheel etc are a possibility considering where you saw the spark. Unfortunately the only practical way to check is to remove the shell to find out.