Ok, I think I’m close to figuring it out, and I believe the problem is not with the ZW. The accessories wiring is led underneath the track near a point where the A loop wiring connects to the track. I think when I was running two trains hard, that section must have heated up enough to melt the insulation on the accessories wire and it came in contact with the track.
I know this wiring looks crude but it is temporary while I finish the layout. This was the run and when I pulled it up, bingo, continuity gone.
So obviously I need new bigger wire for the main accessories run to the bus - any further thoughts on how to prevent this much heat from causing further problems? Is this a common issue that everyone contends with?
Thank you all so much for your help. This forum is awesome!