Pop the top, and look close at the ZW's rollers and winding wires, posts and connections. Deeply worn, or deeply sawed, "grooved" windings are bad news. It limits the transformer strength greatly. Finding weaker VW innards is a possibility too. But either should be able to handle two, or more trains.
Can you run them without lights, and not trip the breaker?
Do you have more than two locos to test with? One may have an issue.
Can the tripped breaker be reproduced with any two tracks hooked to the same two handles? (Third disconnected).
Bad center rail isolators can also cause this without stopping trains, creating heat, or tripping the breaker right away. The weight of the trains running over them causes a temp. short, but it is weakened by motor draw.
Are your circles connected by turnouts? With insulated plastic pins?
Do you have any track blocks?
Are you using more than one power feed to each loop?
