The tightest curved section of Atlas N-scale snap track is 9-3/4" radius. That's equivalent to an 18"radius curve in HO -scale. Both are considered quite tight curves. What do you plan to run on 7" curves? Trolleys would be able to negotiate that tight a curve and some short wheelbase locomotives like a Plymouth switcher, or 0-4-0 steam locomotive. 40' freight cars too. However most cars and locomotives intended for mainline railroad service may not be able to handle such extremely tight curves.