Regardless of the rated power each output can only deliver 10 amps. UL requirement. Doesn't your ZWL have ammeters? 12 passenger cars lit with incandescents are likely drawing 5-6 amps alone. Measure the current draw with just the cars on the tracks. Each motor has a stall current of around 2.5 amps. While its unlikely they are approaching that you can see the motors alone could be drawing 10 amps.
Pete