On my 9 by 12 layout I have 3 independent loops plus a trolley line. I operate the entire layout with 1 post war ZW and 1 TW which was made especially to power accessories. The ZW more than easily powers the 3 loops as I run the engines with cars at the same time with no problem. I will experiment to see if bringing them online 1 at a time actually results in a slow down. I also use the ZW's 4th circuit to power a siding with multiple switches where I store passenger cars just to see them light up. The TW powers the trolley. I would conclude that the ZW is more than acceptable (and is more than 50 years old).
John
