I would try using a couple test leads to hook the ZW to an engine that is laying on its side off of any track by touching the roller with on test lead and the wheels with the other lead to see if a short occurs. Would definitely let you know if the ZW has the problem internally.
Is there anything else possibly powering the track? Accessories you ground via the outside rails, etc. If so, it may be that your ZW is out of phase with the other source.
