Thanks for the tips about the watchdog signal. I think I can avoid this challenge with my yard. The two mainline are taking up all four channels of one TIU (2 per mainline) since I'm using older PW transformers. Split each mainline into two districts.
So the yard will be on a 2nd TIU. Thinking I can power the yard and yard's TIU separately. And, maybe I'll just use the variable channel to ensure no launches at 18V if I mess up the watchdog signal.