Understanding Curves

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?

S Curve

Curves add to the cost of railroading - but also the drama!

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.

O-72 1/2 Curve

You can use smaller sections of curve to increase your trackplan options. This is a 1/2 section of O-72 curve.

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:

curve comparison

Similar locomotives in HO and O share curves of the same radius (18" / O-36). Both curves are very tight, but the O Gauge model requires extra accommodations.

  • 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?

O-36 Curve

The O-36 curve will accommodate a large variety of locomotives and cars in a relatively tight space.

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!

11 thoughts on “Understanding Curves

  1. Dear Folks

    This was very helpful

    My son and I are building a 10 x 9 foot layout.

    The full scale Hudsons we are planning to have request O-42? Should we just go with O-60. That will be our outer line and then use the O-36 the rest of the lay-out.. Will this work?

    • Dear Joseph,
      Glad to hear this guide was helpful. Hopefully as we continue to expand these pages you’ll get more information you can use as you build your layout. As to your question, yes, you can use O-60 on the outer loop and O-36 elsewhere, you just won’t be able to take the Hudsons on those tracks. Depending on what sort of plans you have, you may even be able to go a little larger on the curves. The Hudsons will negotiate anything as tight as O-42, but they will certainly look a lot better on bigger curves. Keep in mind what you want to pull behind those Hudsons too. Many of our scale cars require an O-54 minimum curve.

      Feel free to write back with any more questions as you go along, and we’d love to see pictures of your progress as well! It’s easy to share via our Facebook page and we love hearing from you.

  2. Thank you for such a speedy reply!!!!!!!!

    I an ordering the track Monday and will keep you informed of our progress.

    The Lionel Hudson in Tuscan Red will that handle the O-60 nicely?

    Or should I just go with the O-72 for the outer loop

    Thanks again


    • Are you sure you’re looking at a Hudson and not our Tuscan Red K-4s Pacific? Either way, both will operate equally well on O-60 and O-72. Your trains might be able to take the O-72 curves with a little greater speed, but at that point you are really just dealing with looks. The locomotives, along with long passenger and freight cars, will have less overhang with larger curves. If you do decide to go with O-72, then the one plus is that no matter what you get down the road, you know you’ll have a loop of track that can take it!

      If it were me, personally, I’d go O-72. But I have a thing for big equipment and sweeping curves! You’ve certainly got the room to go with O-72 if you want, it will just be a little trade off between the size of the curve and the amount of space you have left around the edges of the platform. You could cut back the curves a little without any detriment to operations and give yourself a little more room for other scenery. Best of luck and have fun!

  3. Hello, I have prepared a 9.5′ by 11′ dedicated room in our celler for an “O” layout and wondering where to start. My wish list includes some part of the room with a side by side run using Fastrack 72, also 60, and 40 in spurs and working yard. Most of the layout should just be a long run and spurs and a working yard. Thinking of using one or two “lift out bridges” for access to all parts of the room. Plan to have about a 2.5′ entrance to the room area. How likely is this incorporating 72 for the large engines? Any suggestions?
    Thank you,
    Ty Stinson

    • O-72 shouldn’t be a problem in that space, keeping it in an oval around the walls or a large platform in the center. You can get a lot more creative with the plans for the other loops with tighter curves.

  4. An important consideration in laying multi-track curves is the track to track clearance. Be sure there is enough space between curves so that long cars and engines don’t hit one another as they pass. There is always a temptation to squeeze more track in but that can lead to problems, especially with the smaller diameter curves. Another thing you may want to try is what railroads call easing a curve. You can do this by using larger diameter curves at each end of a curve so that your trains ease into the turn rather than hit a sudden 90 degree bend. Adding some short straight sections between curved sections can also make the curves less sharp. Both of these tricks can reduce derailments, allow for longer cars and engines, and make your turns look more realistic. You’ll need to experiment with different combinations of track to get the look and performance you want. I’ve been able to operate a Lionel Reading T1 locomotive on O-31 curves that are eased with 1/8th and 1/4 straight sections with no derailments. The squeal from the wheels on these curves is just like the real thing!

  5. Very good article, one thing to keep in mind. If possilbe at the end of a curve real railroads have a tangent before starting into a curve starting in the opposite direction. The reason for the tangent is to allow the lateral movement and motion of the train to settle before changing directions. in very rare situations are curves placed one after another. That is sometimes seen in an industrial setting, When the industry was built in the late 1800’s or early 1900’s. Another reminder would be try not to place any turnout or switch at the end of a curve. Place a tangent at the end of the curve then place the turnout. There are many things required to properly lay track (real track). I try to use those same standards when I set up any of my layouts that I do from year to year. Also the larger the curve the more real estate with be used. Large radii looks good and I try to use whenever possible.

    I personally enjoy reading these, keep up the great work Lionel!


  6. I use 27in,31in, 042,72in and some odd twists to get a 40ft x 20ft to fit on a 19×19 diagonal layout.It took a year and a half to figure out and I windup with a beautiful SD-40 for 036 minimum. I hope ya’ll never have to move.

  7. I have aluminum passenger train cars (MTH) that are 16″ inches long, what track curves would be a better, more realistic look MTH fast track 042 or 054? New to the hobby and want to do it correct. Thank you for any viable input.

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