I forget the actual measurement, but when I used an HO 40' box car on an 18" snap-track segment, and an HO scale rule, I measured significantly less overhang.
The loving fear of the Bride 390 XVII.
I'll happily process them.
You must learn to deal with compromises in our hobby.
I'd say, for what you describe, and to meet your desire to run stuff with your club, go with the 18" radius.
That works zero percent of the time.