So based on the additional information you supplied for my clarity, I would have to agree with @cjack that is sounds like a faulty circuit breaker in the ZW. If you look at the ZW Schematic below, within the red box, you will see the circuit breaker in parallel with the short circuit lamp/resistor. The circuit breaker and lamp/resistor are connected from the U terminals to the ground of the winding, thus all the current out of the A-B-C-D terminals must flow back into the U through the circuit breaker. When the circuit breaker opens due to a short, the current must now flow through the lamp/resistor, which severely limits the current, but enough to light the lamp. When the circuit breaker cools, it closes again, bypassing the lamp/resistor, and is ready to protect again.
The circuit breaker is supposed to handle around 10 amps, but your ZW's circuit breaker seems to be opening a lot less current passing through it.
Try an illuminated car (or two) of some type, and see if the ZW can handle that. Just trying to get a better idea that it is a faulty breaker and not something else.
