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Model Train Track Curves and Minimum Curve Issues


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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

Author's photo (Click to enlarge).
A radius in geometry is the distance from the center of a circle to any point on the circle. 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 those shown.

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

Layout diagram by author (Click to enlarge).
Track curves can be used to break up the monotony of long straight sections of track. But their real utility is realized in turning your trains around. The rule of thumb in model railroad layout construction has always been to use the largest radius curve that you can. One of the signs of model railroad addiction is "You look at a table or other flat surface... and start estimating the largest radius curve you can put on it." I agree that you should use curves with a radius of 18 inches or larger in HO and 11 inches or larger in N scale... if you have the space.

The Problem With Narrow Spaces

So 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

Table by author (Click to enlarge).
The smallest space a train can turn in is its minimum radius. When choosing a scale in model railroading it is important to remember that the larger the scale, the larger the minimum radius for your curves will be. The chart at the left shows the minimum curve available in various scales from different model railroad track manufacturers. Click the link to read it.

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

Image courtesy Plaza Japan (Click to enlarge).
After writing this article I learned that the Japanese manufacturer Tomix offers N scale minimum curves of 103mm radius, or 4 inches. Serious prototypical model railroaders regard Kato's 8.5 inch radius track, as much too small for their layouts, so they certainly won't take Tomix' 4 inch curves seriously. I think that for children, if you ensure that their trains can handle these tight turns, these Tomix curves will allow them to be much more creative in laying out track on an under-the-bed board. However, Tomix track isn't readily available in the US, as Kato track is. It can be purchased online from Plaza Japan. If you know of other sources for Tomix track in the US, please notify the guide so that they can be listed here.

Locomotive Size Is An Issue

For 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 Track

If 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 You

If 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.

Easements Are Another Option

An easement is a track laid in a parabolic curve. Usually easements are made with flex track, but they can be simulated with segmented track pieces. Easements will increase the width of your turns a bit, but they will also make your layout look a little more realistic. For more see my article Model Railroad Track Easements.

Next Stop, Making the Grade

Now that you know the basics of curves, including the implications of tight minimum curves, the next concept you need to familiarize yourself with is the issues surrounding steep maximum grades.
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