If you’re just getting started building a model railroad, understanding curves can be a challenge. O-27, O-48, radius, degree, arc – how do you make sense of all of this without retaking Geometry?

First, a little background on curves and how they’re measured. In a perfect world, railroads would build their tracks in a straight line. Curves increase friction and wear on wheels and rails, and cause reduced train speeds. But of course, the world isn’t flat, and even our train platforms have edges so curves are a fact of life for all of us. On the prototype as well as our models, trains run faster and safer through larger curves, but there is always a need to compromise whether it is a mountain or the end of the plywood.

Given the large scale of real railroads, prototype curves are measured in degrees. For modelers it is usually far more practical to measure our curves by their radius or diameter. If you’ll remember your basic geometry, the radius is a measure from the perimeter of a circle to its center. The diameter is twice this distance, ie. perimeter to perimeter through the center.

In most scales, radius is used as the standard measurement and pre-made track sections are available in several common radii. With O gauge, diameter is often used when measuring pre-made curve sections. These curves are shown as O-xx, where “xx” is the diameter of the curve. O-27 is the tightest curve available and has a diameter of 27 inches. O-27 can also be used to identify the profile of the rail which is higher than scale to better accommodate the needs of model trains. The term is also sometimes applied to all trains and tracks that operate on anything other than true O scale. Most Lionel sets include O-36 curves.

When measuring track, the radius and diameter correspond to the centerline of the track. In other words, the actual outside dimension will be slightly wider than the designation. In other words, a circle of O-36 track will not actually fit on a 3 foot square table. When using FasTrack, it is generally a good idea to add an additional two inches to each side (so 40 inches for a 180 degree turn) to accommodate the roadbed. If you are running longer cars and locomotives, additional space on either side of the track may also be necessary to allow for overhang.

How Tight Are Model Curves?

Just how tight are the curves on our model trains? Very. Here are a few facts for comparison:

• O scale trains are twice the size of HO trains. The standard train set curve in HO scale is 18″ radius (considered tight by HO standards) – the same as our O-36 diameter.
• Many true O-scale 2-rail layouts use 72″ radius as a minimum curve (not O-72) – in other words a circle of track requires a room slightly larger than 12′ x 12′.
• The curves of the Pennsylvania’s famous Horseshoe Curve (one side of the curve is actually a little sharper than the other) were among the sharpest on the railroad. If laid out in O scale to an accurate dimension, a model of the curve would be about 33 feet wide.

With numbers like these, it is easy to see why most O gauge modelers have accepted the compromises of curves that would be too tight for most of the equipment we model.

What Size Curves Should I Use?

The best minimum radius for you is a matter of personal choice. Obviously, trains will look more realistic on larger curve sections. If you want to run models of larger, scale length equipment and large locomotives then you may need to use O-48 or even O-60 curves or more.

As a general rule, it is a good idea to use the largest curves possible in the space you have available. There are a variety of track and layout designs that make curves less of an issue. For example you could build around the walls of a room instead of a platform in the center, or build a linear switching layout. Many modelers place a loop of the largest curves they can around the perimeter of a platform dedicated to the largest equipment and run shorter cars on more elaborate track plans within the center.

Like most aspects of the hobby, there are few strict rules about what is right. After all, most of the prototypes for our trains would just keep going in a straight line if they ever encountered any of our curves. Just do what is right for you!