I have experimented with Kato N scale Unitrack down to 4.7 inch radius, designed for tram stuff. There is N scale freight equipment that will actually "work" reliably at that radius, where "work" is defined as pulling (not pushing) 3-4 40' freight cars slowly around a semicircle turnback curve. The key is either traction tires or a lot of weight on the engines, since all wheels will be sliding as they roll. As I recall, the best engines were the Rapido SW-1 with traction tire or (don't laugh) a massive A-B-B-A lashup of Life-Like FA1/FB1 units. I don't recommend this for general operation, obviously, but for, e.g,, a hidden turnback on a shelf layout it's something to consider.
If you increase the radius to 6", many other short 4-axle units can "work" in this limited context, GP-35 size or smaller. At 7.25" radius, almost any 4-axle loco will work, and even some 6-axles without excessive binding. I have noticed that older N scale locomotives were designed with looser fit than some newer ones, and are more forgiving of sharp radii. Likewise floppier couplers help (Microtrains and Kato vs.stiffer Accumates).