corp5382;
Again, it really depends on what you want to run.
Scale Steam engines and 72' passenger cars are among the worst combinations.
A scale Big Boy has insane boiler swing out on O-72 and can't even take O-54 curves much less O-42.
Adding a few inches of straight on the outer loop sides before starting the curve helps avoid the issue of close curves.
I suggest you get a curve of each size, set them up side by side and test by hand with your longest units. one on inner curve, one on outer, then swap them. If you have over 1/4" gap at the closest point I'd expect you are OK.
