Last Updated on September 11, 2022 by Dave Farquhar
When it comes to O gauge trains, I don’t know if there’s a more confusing topic than track dimensions. Standards, schmandards. Let’s see what I can do to de-mystify O gauge curved track dimensions.
O gauge track comes in some common dimensions, the most common being 31, 36, 42, 54, 72, and 96 inch diameters. The problem is not every track system has each diameter, and the measurements tend to be nominal, kind of like lumber.
Measuring O gauge curved track dimensions
The reason O gauge curved track dimensions are nominal is there is no good reason. Unlike lumber, which starts a standard dimension and ends up smaller after the factory finishes smoothing and planing it, no one can agree on how to measure O gauge train track. Is it center rail to center rail? Is it outer edge to outer edge? Outer rail to outer rail? Do you round it off?
It can even vary from production run to production run. True story: I once bought a circle of 54-inch diameter track. I didn’t use it right away, so I didn’t notice that I didn’t buy a full circle. So I went back to the same store, a few months later, and bought the same thing. I brought it home and noticed the new pieces were a good 1/8 inch shorter than the old ones. It was close enough that it still worked. And I could account for the difference by a switch from measuring to the outer edge to the outer rail. But supposedly identical pieces of track were hardly identical.
And I can’t even account for the variance being due to brand. Yes, two different brands will probably be slightly different. But this was all Lionel track.
Planning for variances
So while everyone seems to manage to get the lengths of straight tracks right, regard the curved track dimensions as approximate. I strongly caution you against building an 8×8 table and putting a loop of O-96 track right up to the edge of it. First of all, there’s the risk of your train falling off the table if you don’t leave a few inches of slack. But second, if they measured center to center rather than edge to edge, that O-96 circle might hang off the edge.
To be safe, always step back one size from the maximum for your outer loop. Use O-72 on an 8×8 table. You can pad it out with straights to fill the area more completely while leaving adequate slack space between the edge of the track and the edge of the table.
Now, if you’re following a track plan in a book, and you use the same track system the author did, it should work. Especially if you use a modern track system that includes roadbed. Changing dimensions on a whim is less likely to happen with roadbed track because you have to retool to account for the roadbed. But if the plan calls for Lionel Fastrack and you build with Atlas track because you like Atlas track better, don’t be surprised if you have to make some minor adjustments.
Common O gauge curved track dimensions
Not every track system has every track dimension, but most track systems will have the majority of them, or something pretty close. The most common nominal dimensions follow:
- 27 inches (8 to a circle)
- 31 inches (8 to a circle)
- 36 inches (8 to a circle)
- 42 inches (12 to a circle)
- 54 inches (16 to a circle)
- 72 inches (16 to a circle)
- 96 inches (16 to a circle)
Note that the number of pieces to a circle varies. The larger number of pieces to a circle allows more gradual changes with the larger track sections. It also allows you to transition into sharper curves. Rather than going around the corner of your table with a sequence of straight-O31-O31-straight, you can go around the corner with O54-O31-O31-O54. It takes a similar amount of space, and reduces derailments. You can even shorten up your other straights and use O72 or even O96 in place of the O54s if you wish.
As for why O42 uses 12 sections to a circle making it difficult to transition in and out of it without cutting curved pieces, it’s because Standard Gauge track came that way, and when Lionel introduced O42 track, they didn’t change the geometry. And you know how some people can be–once you do something twice, it’s tradition, and you have to do it that way forever.
I’ve also compiled information including the lengths of straights and the pin sizes.
David Farquhar is a computer security professional, entrepreneur, and author. He started his career as a part-time computer technician in 1994, worked his way up to system administrator by 1997, and has specialized in vulnerability management since 2013. He invests in real estate on the side and his hobbies include O gauge trains, baseball cards, and retro computers and video games. A University of Missouri graduate, he holds CISSP and Security+ certifications. He lives in St. Louis with his family.
Missing from your discussion is pre-war American Flyer O. It is still available out there and has a 40 inch diameter outside tie to outside tie. That puts it 1 inch wider than the outside diameter of Lionel’s current O36 Fastrack circle which measures 39 inches across.
Oops. I forgot that the whole reason for bringing up the pre-war American Flyer O is that it required 12 sections to make a circle. So, there may have been an O gauge precedent behind 12 sections for an O42 circle other than standard gauge.