Then multiply by the radius to find the length of the arc. To find the angle, divide by the radius. That gives you the angle in radians. That can be converted to degrees to get Crawley's answers. So, to find the radius r, first convert the angle a to radians, then divide that into the length l of the arc.
Arc length equals radius times the angle in radians.
It helps to draw the figure. The radius to the outer rail is 2102.5 while the radius to the inner rail is 2097.5. Convert to radians. The radius is 3956. Since the length of the arc equals radius times the angle in radians, it follows that the angle in radians equals the length of the arc divided by the radius.