For example if you look ar the Aristo-Craft site the Pacific 4-6-2 www.glacierwa.com/trains/pacifics.jpg requires a 6.5' diameter curve, this is the diameter of the track, the degree of the track is the degree that each section covers out of 360 degrees. The Aristro 4' diameter curve www.glacierwa.com/trains/track.jpg consist of 12 30 degree sections and thus the Pacific would not run on it.
I hope that helps.
I hope that helps.