Available diameters of tubular O and O27 track

A frequent question, especially for those who are just discovering or rediscovering vintage Lionel and Marx trains is what sizes of track are (or were) available, and how many pieces come to a circle.

Unlike other scales, Lionel marketed its track by diameter, not radius. As you undoubtedly remember from geometry class, radius is the distance from the center of the circle to the edge, while diameter is the distance from edge to edge. So a circle of O27 track is approximately 27 inches wide. O27 track stands about 7/16 of an inch tall, while higher end O gauge (also sometimes called O31) track stands about a quarter inch taller, at about 11/16 of an inch tall.

While we’re on the topic of track, here are some tips for connecting track if your new track isn’t going together as easily as it could, and some tips for screws to attach track to the table.

Here are the available sizes, in ascending order.

O27 track

We’ll start with available sizes of O27 tubular track. These sizes all stand about 3/8 of an inch, so you can use them together. Modern-production O27 track has brown ties, making it easier to distinguish from O31 track, although postwar O27 track also had black ties so tie color isn’t always a reliable indicator. Here’s a collection of all the relevant dimensions of O and O27 track.

Many hobbyists turn their nose up at O27 track, but you can easily run large trains on O27 track with wider-diameter curves. While both track types are too large to be scale-sized, the lower profile of O27 is closer to scale than O31.

O27

This size, 27 inches across, was supplied with entry-level trains for decades. They came 8 curved pieces to a circle, so each piece measures 45 degrees.

O34

This size, 34 inches across, was developed by Marx to use in its higher-end sets to better compete with Lionel. Production started around 1952 and ended around 1972. Marx O34 track also came 8 curved pieces to a circle, measuring 45 degrees. The easiest way to identify O34 track is to look at the ties–a piece of O34 track has five ties, when other types of O27-profile curves almost always had three. O34 track always had black ties.

Marx’s track pins looked a little different from Lionel’s, which sometimes causes confusion, but the two types of track are completely compatible. The Marx pins mate with Lionel track just fine, and if a piece of Marx track is missing pins, a replacement Lionel pin will fit.

O42

This size, 42 inches across, was produced by both K-Line and Lionel starting in the 1980s. O42 was a bit of an oddball size in that 12 pieces go to a circle, with each piece being 30 degrees. This limits your options when using O42 track in a loop with other sizes. Since O42 track is modern production, it has brown ties.

O54

This size, 54 inches across, was also produced by both K-Line and Lionel starting in the 1980s. It has 16 pieces to a circle, with each piece measuring 22.5 degrees. Here’s a neat trick: You can make a 90-degree curve with a sequence of O54-O27-O54, and it will take a comparable amount of space on your table as a sequence of straight-O27-O27-straight, but since your train gets to ease into the curve, it will derail less and you can operate it at a higher speed if you wish. Since O54 track is modern production, it has brown ties.

Japanese O72 track
This O72 track looks like Marx but I think it was made by Sakai or another Japanese company due to its use of Lionel-like pins. The pins are wrong for Marx and the tie spacing is wrong for Lionel.

O72

K-Line produced this size, 72 inches across. Unfortunately they went out of business in 2005. This track was never super common and has been out of production for 10 years. It also had 16 pieces to a circle, with each piece measuring 22.5 degrees. If you’re lucky enough to locate some O72 track, you can use the same trick as above with it. Since O72 track is modern production, it has brown ties. The catalog number for this, if it helps, was K-0226.

Additionally, some postwar-era production O72 track surfaces from time to time. I think Sakai or another Japanese company made it, as neither Lionel nor Marx did. Superficially it looks like postwar-era Lionel or Marx track but the tie spacing isn’t quite what either company would use. The characteristic that points away from Sakai is the use of brass pins shaped like Lionel. Neither Marx nor Lionel produced track of this diameter in O27 profile during the postwar era. When I first saw this track, I thought someone had put postwar ties on K-Line track, but K-Line put the ties closer to the ends than whoever made this track. And, while uncommon, this track surfaces too frequently to be someone’s basement special.

The easiest way to pick O72 track out of a lineup is to look for six ties to a section, and match the look of the ties to the track you already have.

O31 track

You can quickly identify O31 tubular track by its black ties. This profile of track measures about 11/16 inch tall.

O31

This size, 31 inches across, was supplied with entry-level trains for decades. They came 8 curved pieces to a circle, so each piece measures 45 degrees.

O40

This size, 40 inches across, was developed by AC Gilbert for its 3/16-scale O gauge trains that it sold from 1939 to 1942. It used 12 pieces to a circle, measuring 30 degrees. It is indeed slightly smaller than modern O42 track.

O42

This size, 42 inches across, has been produced by Lionel and other manufacturers off and on for decades. O42 was a bit of an oddball size in that 12 pieces go to a circle, with each piece being 30 degrees. This limits your options when using O42 track in a loop with other sizes.

O54

This size, 54 inches across, was also produced by by various manufacturers for many decades. It has 16 pieces to a circle, with each piece measuring 22.5 degrees. Here’s a neat trick: You can make a 90-degree curve with a sequence of O54-O31-O54, and it will take a comparable amount of space on your table as a sequence of straight-O31-O31-straight. But since your train gets to ease into the curve, it will derail less and you can operate it at a higher speed if you wish.

O72

This size, 72 inches across, is the favored minimum diameter for operators of modern production Lionel trains. Full 1:48 scale O gauge trains look and work best on this diameter track or larger.

It also has 16 pieces to a circle, with each piece measuring 22.5 degrees. The same trick above for transitioning into smaller track works with O72 as well, if you don’t have room for a full O72 loop, so you can still get some benefit from it even on a small layout. You can go around a curve with a sequence of O72-O31-O72, or O72-O54-O54-O72.

Sometimes trains intended for wider diameter track can make it around one of these hybrid curves, allowing you to run scale trains on a semi-scale budget.

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