Ah, a nice brass engine I presume. I only have a few Bachmanns that are not as finicky (and not as nice). I tried to click on your photo to be able to zoom in on the plan, but it does not allow me to do that for some reason. It looks OK from 30,000 feet above .
Simon
My On30 K-27 requires 26".....
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The big difference I find is with the buildings. My HOn3 and On30 engines all require 18" radius curves.
I have updated my track plan from HOn3 to On30.
It sure eats up more real estate in On30 compared to HOn3..