Shaun, Bill Kamery is correct, there is no true radius in real railroad practice. In real life super elevation curves are parabolic in curvature and the outside rail is elevated higher than the inner rail. Keep in mind that the BigBoy 4-8-8-4 was designed to take a 20 degree curve, and some narrow gauge engines could actually take curves as tight as 28 degrees! When you figure that out in "N" scale, you will see where a 15" radius falls. HA!
