Model Railroad track curves may seem like a simple issue, but there can be much more to them than meets the eye. Track curves on model railroad layouts require some planning and knowledge of a few relevant issues. People new to model railroading are sometimes unaware that model train track curves are sold in various radii. Modelers frequently like to run tracks parallel to each other, and this requires curves of different radii. So what's all this about radii?
Track Curves Radius and Arc
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. This is the arc necessary to turn a train around. The curved pieces in the 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.
Track Curves Are All About Space
The Problem With Narrow SpacesSo what if your available space is 2 x 5 feet? Many experienced modelers would tell you that you can only build a switching layout, a layout with no 180 degree turns, in such a space. On switching layouts your trains can only run back and forth, and modelers simulate setting out and picking up cars from industries and connecting up cars to make a train on them. Some people really enjoy doing this.
But if you really want a continuous loop layout in your small space, despite the fact that it may not look prototypical, then minimum radius becomes very important to you. Don't let someone else's rule of thumb ruin spoil your fun. I've said a lot more about space in my article on train tables and boards for children.
Minimum Radius and Diameter
In geometry, the diameter is twice the radius. But when planning space to turn your trains around, you need to remember that the radius given by the manufacturers is measured from the center of the track, not the outside edge. So you need to add the overall width of a track piece to the diameter in order to properly calculate the space needed to turn a train around.
N Scale Note
Locomotive Size Is An IssueFor navigating tight turns your primary issue is the wheelbase of your locomotives and rolling stock. The tightest curve Kato manufactures for their N scale Unitrack has an 8.5 inch radius. This means you can fit an oval of Unitrack in a space as tight as 18 inches, or a double track in an area with a width of about 22 inches. However, Kato warns you that six axle locomotives won't be able to navigate the 8.5 curves. As long as you're aware of this fact, you won't waste your money buying big modern diesels or long articulated steam locomotives and face the disappointment of them derailing on your curves. Short trains pulled by smaller steam locomotives, or short cuts of modern cars pulled by a switcher look fine on these curves.
Don't Be In a Hurry to Glue Down TrackIf you're new to model railroading, don't be in a hurry to glue down your track and start landscaping. Throw out the buzzwords "prototypical scale model" and remember that these are toy trains. So play with them. Change your track around and experiment. Discover what works and what doesn't... and what you like and don't like. Use a segmented track for this... preferably one with an integrated roadbed. If and when you go to build a permanent layout you'll likely want to change from segmented track to flex track, but everything you learn from segmented track about curve radius and arc will still apply. I don't think you'll be unhappy with your investment in the integrated roadbed track... unless you bought a number of expensive turnouts.
Camouflage If Un-Prototypical Bothers YouIf the look of tight turns is going to bother you, and you don't have space to widen them, you can camouflage your curves with tunnels and narrow canyons. For a great example of this take a look at the PRR layout by Dave Vollmer. Dave built this layout on a 36 x 80 inch door. I'm not saying Dave's curves are too tight, I'm saying that even if by some people's standards they happen to be, I can't see it. In any case, this layout doesn't look like a double track oval.
If you hide tight curves, don't forget the limitations imposed by them. Camouflaging doesn't let you run six axle locos, it just makes the trains that you can run look more realistic. In any case remember, never let other people's opinions spoil your fun.