I have seen ZW's where the carbon roller is actually worn away completely. In this case, contact relies on the copper arm that held the roller rubbing on the transformer coil. I imagine this results in frequent loss of power. You didn't say, but I assume the problem handle in the big handle on the right. Since most boys are right-handed, that handle gets the most work. Usually on the old ZW's, the rollers on the two small inside handles are nearly new since these handles don't get moved much.
Since you are only using the ZW for track power, I recommend powering your two loops using the inner "B" and "C" outputs.
