post war lionel zw transformer

I can't remember if the carbon rollers can do that. If they are worn down.

Are you sure there is a short?  Do lights dim, things get hot and start to smell like burned insulation?  A common failure for the ZW is for the circuit breaker to start opening for no reason. Replacement circuit breakers are available. 

As Riki suggested, it would not hurt to check the carbon rollers. With the ZW unplugged, remove the two lamp covers and the two light bulbs, then the four screws on the top cover. It will just lift right off.  The four carbon rollers should turn, the copper bracket should not be touching the transformer windings, and the arms should not touch each other as they pass. If rollers need replacing there is lots of information on how to do on this web site. Use the search function to find it. 

Thanks for the advice.  I'm guessing it is the circuit breaker from the looks of things.  Just ordered a new one.  Thanks again.