Why I always test my engines on the mainline, going east and west.
Testing the new UP 4-6-6-4 on the mainline, the engine and cars ran fine going East but West, the engine was shorting at one spot. Not sure if cars or engine so separated the two and the engine was run very slowly and it shorted at the turnout. Looking closely, one can see the air hose touches one of the turnout points.
Air hose is hanging too low: easy fix.