I had something similar with an MTH Z-1000 brick. The polarity was reversed with respect to the pair of accessory terminals and the barrel connector. If you wired accessory common to track ground (outside rail) and then wired the Z-1000 Throttle such that black went to common (track outside rails) and red went to the center rail the breaker would instantly trip when you plugged the thing in. I finally cut the wire and spliced it back together reversed.