After 25 years, I have a conductivity problem. I removed axles to clean and grease gears. I carefully re-assembled and now it's not running. Power Pack trips off, direct short.
No loose wires observed , I'm thinking it's something with the non drive wheels power pickup. May have remove trucks.
L.p.The last reply probably is the case. You'll have to identify which of the trucks is meant to feed which post on the decoder, and then make darned double-sure the pickup end of the axle is under the wiper, or against it, for each of the axles in that truck. If you have all axles correctly inserted, all wipers making contact, and you still have a short, remove loco and restore track power. Does it still want to stop a short? Then it's not the locomotive.
One other remote possibility is that the insulation has been worn over time inside one of the inner wires and it is allowing contact with the frame or some other wire and you're now shorted.