You can use all kinds of mathematical formulas to calculate the radius of the curves on your railroad, but there’s a much simpler way to do it in the field.

Start by cutting a 10-foot length of string that has no stretch. Secure one end to the inside rail of the curve, and pull it tight so the string forms a straight 10-foot chord across the curve.

Next, measure the sagitta — that’s the distance from the center of the string straight down to the inside edge of the rail.

Once you have that measurement, simply refer to the included chart to determine the radius of your curve.

Download the chart [Track Radius Chart here].