What is 'arc' on a turnout?

When choosing a turnout or curve for your model railway, you may have noticed an 'arc' measurement alongside its radius. But what does this mean?

An arc is defined as a part of a circumference. When used as a measurement on a model railway track, an arc is a way of expressing the degree of course change the track makes. So a curve or turnout with an arc of 45 degrees will result in a route diverging from its original heading by 45 degrees. Connect two tracks with an arc of 45 degrees and the route will turn at a right angle (90 degrees). Connect eight 45 degree arc curves and the route will turn a full 360, and so on.

The 'arc' measurement is particularly useful for modellers trying to calculate which sections of track are required to achieve desired track plans. The most common arc calculation used by railway modellers is when attempting to return a track's route to straight ahead after a turnout. In this instance, a turnout with an arc of 45 degrees will simply require a curved section with a matching 45 degrees arc to get back on course with a heading parallel to the primary route.

Arc also helps modellers understand how many curved sections make up a complete circle of track. This is especially useful when using Setrack with larger radii as these tend to have smaller arcs.