Look at the underside of the loco. If you see any blackened, or "bubbled" areas on the rollers or wheels, it indicates that there was heavy sparking or arcing, most likely a short circut. Long-term, this will cause pitting and oxidation so it's worth a little trouble to figure it out.
You might use an ohmmeter to verify that both center rail rollers are connected to each other, picking up electricity, and not shorted to ground. Also, that the rollers are properly insulated from the truck block, all screws are snug, and no loose or broken wires are causing short circuits.
Wild guess, but do both electrocouplers work as they should?
