A Beginner’s Guide to Curves

11 01 2012

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!



15 responses

11 01 2012
Joe Partsch

I have read a few articles and still confused. I recently bought the O-Gauge Polar Express that says it has a min of 027, we are looking at Hallow Eves Express which states min of 036. Does the track that comes in the Polar Express make a 27 or 36 curve. I have four pieces of fast trak curve that makes our turns. Thanks for any input

11 01 2012

The FasTrack in the Polar Express set is O-36. A quick way to confirm this is to assemble a circle of track and lay a yardstick across it. An O-36 curve will be wider than the yardstick (on account of the 36″ being the diameter at the centerline of the track.) Even though the Polar Express train will negotiate a tighter curve, we include O-36 with all our FasTrack sets. Modelers with older O-27 curves can still use this train too however.

11 01 2012

yes, I have 8 independent track lines.. The three inside curves have Fastrack 0-36 curves and the five outside curves have Fastrack 0-48 curves. The layout is set up on a 10 feet wide by 11 feet long two tier board..Thanks LIONEL for introducing Fastrack.. It would be great to discuss wiring track and accessories because as I add more lighted cars, more accessory uncouplers, signals , train watch tower, LIONEL Depot station,beacon, switches , and other motion.,I tend to loose power in certain areas of the layout and the trains slow down. My three grandson enjoy LIONEL TRAINS just need to improve the quality of action.. Tom S GEORGIA

11 01 2012

Thanks for the suggestion Tom! We will get to wiring and accessories too, along with scenery and lots of other fun projects.

11 01 2012

Great articles… please continue!

12 01 2012

Can you explain why the curve diameters were chosen, e.g., 36, 48, 60, 72 or 31, 42, 54, 72? It certainly appears that the track spacing for nested curves could be a lot closer together, yet still allow large overhang (long) cars to pass without hitting each other. Even with the better looking ballasted sectional track now available, it’s hard to create a realistic looking layout because of the almost mandatory wide spacing, especially for yard sidings.

12 01 2012

As you suggest, overhang compensation for longer cars and locomotives is part of the reason. The other primary factor concerns the geometry of the turnouts. Since these replace a standard curve section of the same diameter, a crossover between two consecutive loops would leave you with this spacing.

12 01 2012

I think this makes sense. Are you saying if I had a 42″ curve oval evenly spaced inside a 54″ curve oval, I could connect the two tracks using one 42″ turnout and one 54″ turnout? Ditto for connecting 31″ and 42″ loops using 31″ and 42″ turnouts? If so, this could make for an interesting track plan upgrade for me.

12 01 2012

Talking to our product development department about it, you would want to use two of the same turnouts to keep the mainlines parallel. (This would probably also look and operate a lot better.) A pair of tighter curves may yield a closer track spacing than the wider radius. Since there are differences between O-27, Fastrack and of course other brands, and without having built a layout like this personally, that’s about as exact an answer as I can give you right now. As I get a chance to talk to some more modelers, if I learn anything more I’ll get back to you.

19 05 2013
Daniel Marlowe

Can I run a original 700e scale Hudson on Fastrack Safely? With its original scale wheels?

20 05 2013

Yes. You should have no trouble.

14 02 2014
Harry Hill

So if I need four curves that complete a circle at a radius of 16″ what fastrack sections do I need for the four 90 degree turns in the corner of the ceiling.

17 02 2014

We don’t make a 16″ radius curve. O-31 (15 1/2″ radius) curves will be your closest bet and the tightest curve you can get in FasTrack. It takes 8 curve sections to complete a circle (2 per corner.) When choosing trains for your loop, make sure they are rated for O-31 curves.

18 08 2014

I am wanting to put a train up on my sons walk. The room is 10′ 4″ x 11′ 4″. Whre should I start? The wood bracing I can do, eletrical I can do. But I have never had a train set before so I don’t know how much of what to buy

19 10 2014
Jim Peters

I Have An MTH Real Track O Gage And I Want to Know Will O 48 Fit Comfortably on a 4 by eight sheet ? Equiv. To A 22 Radius HO?

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