Years ago I bought a couple of HO flextrack curve template thingies. They were about 3 inches long, stamped out of sheet steel, and fit inside HO flex track to help conform the track to a desired radius. You tacked down the flex at one end, put the template inside the rails and slid it along the curve as you tacked it down.
I think one was a 26-inch radius and one was a 28-inch radius.
I've searched the Web and cannot find them. Anybody know the company that makes (or made) these? Anybody know if such a thing exists for N-scale? I know MRL (MLR?) makes a "straight tool" but apparently not curve tools.