I believe the Z1000 is rated at 6 amps. Assuming the Z1000 is operating correctly, something is generating 6 amps or more of current that trips the breaker.
You say you are only running one loop and one engine, either command and/or conventional, which should not be enough to trip the breaker. I would start by re-checking all your track connections one last time and use a multi-meter to verify no shorts or grounds and good connectivity.
If that checks out, I would set up a separate test track of one or two sections of track and hook up the Z1000 to just that section with an engine on it and see what happens. If it trips, I would try a different engine and see what happens.
If all that checks out, I would substitute a different transformer and see what happens.
