I was pondering how to convert the radius of a curve from the prototypes measurement in degrees to the model railroad measurement of inches. I wondered if there was some formula to convert degrees to inches. I read in Model Railroader's book on Mid-sized and Manageable Track Plans where the Montour RR mainline had curves of 6-7% which the author, lain Rice, likened to 24 inches in HO. Horseshoe Curve near Altoona, PA is two curves of 637 feet and 609 feet radii and is listed as having a curvature of 9 degrees 15 minutes. I wanted to convert these to approximate N scale measurements, So I googled it because, after all, google is your friend. Turns out there is a formula to convert one to the other and it is as follows: R = 5,729.651/D where D = degree of curve and R = radius in feet There is also a table of curves from 1 degree to 90 degrees here: http://www.railwayeng.com/Track/Curves.htm Turns out that Horseshoe Curve radii in N scale are 47.75 inches and 45.675 inches while the Montour's alleged 6-7%, which the book says approximates 24 inch HO radius, scales out in HO to about 120 inches. A 17 degree curve according to the table would scale out to 44 inches in HO and 24 inches in N scale while a 33 degree curve would scale out to about 24 inches in HO and about 13 inches in N scale.