We have a Gargraves O-72 curve in a tunnel and the GGD Super Chief cars go through fine. However, the visible curves on the primary mainline are 48" radius (O-96) or larger. For appearances, full scale cars look better on 60" radius (O-120) or better. All Nation gave me a formula that I tend to take to heart -- measure the wheelbase of the car (bolster-to-bolster) and multiply that by three and that yields the minimum mainline curve the car can normally handle at normal speeds. And if you look at an 85-foot (21 inch) passenger car, the wheelbase is right around 16". While they can make it through 36" radius, even with body-mounted couplers, running at high speed can cause problems (not to mention inside/outside clearance problems).
Bottom line -- wider is better. Even on this switching design for 40-foot cars and a single Geep I went with 42" radius on the corner just in case I needed to check a passenger car or six-axle diesel.
