Jim
The tender wheels should be front truck insulated on one side and the rear truck insulated on the other side. Not on the same side! Another thing could be that you do not have a decoder installed in the tender. Your circuit breaker could be seeing the non decodered loco as a short instead of a load.
Pete
I pray every day I break even, Cause I can really use the money!
I started with nothing and still have most of it left!
I'd still bet someone goofed the assembly and installed a wheelset or more the wrong orientation.
But, if that is well and truly not so, then there is a stray wire or a bit of solder on the circuit board of the decoder where it shouldn't be. It is a bridging problem.
Or, possibly stripped insulation letting a wire touch metal.
Or a decoder improperly connected. One end of one wire, possibly both, is in the wrong place?
-Crandell