In geometry, a radius is the line segment from the center of a circle to any point on the circle itself. Smaller circles have shorter radii (the plural of radius is radii). The image at the left shows curved sections of Kato Unitrack in four different radii. Kato actually offers curved sections in seven radii, one smaller and two larger than the radius shown here. The term arc refers to the segment of a circle and is expressed in degrees. An entire circle has an arc of 360 degrees, so half a circle is a 180-degree turn—the arc necessary to turn a train around to face the opposite direction. The curved pieces in this photo each have an arc of 45 degrees. Depending on scale and manufacturer, track pieces may be found in 15-, 22.5-, 30-, and 45-degree arcs. For best realism, the rule of thumb in model railroad track layout has always been to use the largest radius curve that you can. For experienced model railroaders, this is an automatic reaction—to look at a table and other flat surface and to instantly begin thinking about the largest radius curve that will fit on it. Generally speaking, use curves with a radius of 18 inches or larger in HO-gauge trains, and a radius of 11 inches or larger in N scale—if there is space, that is. For those who really want a continuous loop layout in a small space, despite the fact that it may not look prototypical, then minimum radius becomes very important to you. Despite the traditional rule of thumb, there is no reason you can’t create a full 180-degree turn, provided the minimum radius makes it possible. And when introducing a child to the fun of model railroading, rules aren’t the most important thing. In geometry, the diameter is the line segment that bisects the entire circle and is therefore twice as long as the radius. When planning space to turn your trains around, remember that the radius given by the manufacturers is usually measured from the center of the track, not the outside edge. This means that adding the overall width of a track piece to the diameter is an important part of properly calculating the space needed to turn a train around. For children being introduced to the hobby, these tight curves may allow them to be much more creative in laying out track on an under-the-bed board—assuming, that is, that the train cars (especially the locomotive) can handle the turns. Make sure to consider this fact when matching locomotives and other rolling stock to your track layouts. Don’t waste your money buying big modern diesels or long, articulated steam locomotives, only to face the disappointment of them derailing on curves. If a track configuration is limited with tight, short-radius curves, short trains pulled by smaller steam locomotives, or short lengths of modern cars pulled by a switcher, will work fine. Use a segmented track for this experimentation—preferably one with an integrated roadbed. If and when you get around to building a permanent layout, you may want to change from segmented track to flex track—but everything you learn about curve radius and arc will still apply. When hiding tight curves, don’t forget the limitations imposed by them. Camouflaging doesn’t allow a run for six-axle locomotives—it just makes the trains that are able to run look more realistic. In any case, remember to never let other people’s opinions spoil your fun. Easements will increase the width of turns a bit, but they will also make the layout look a little more realistic and operate more smoothly.
