Atlas O Scale Track Size

You are not alone in your confusion. I often get requests for an "O72 Radius" when the customer means diameter. Normally, a track size with an O in front of it is a reference to the old Lionel terminology. O72=36" radius; O54=27" radius etc. Yes, O27 is 13-1/2" radius!

Don Cardiff Model Railroad Design Kaneville, IL

Thanks Don!

-- John

an "O72 Radius"

an O in front of

Don,

You have to be careful with this when dealing with Atlas-O track and plans for "traditional" tin-plate O track.

The diameter of "traditional" tin-plate track is measured from the outside edge of a tie to the opposite outside tie edge. Which means the radius to the track center is actually smaller than you'd think at first glance.

Atlas-O track diameters are measured from track centerline to opposite track centerline, the same as radius is measured with scale track, resulting in a slightly larger circle than you'd get using tin-plate track of the same designation.

Put another way, you have to add 2-1/4" to the Atlas-O circle designation if you want to measure it using the "traditional" method (outside edge of tie to opposite outside tie edge).

So Atals-O O27 track becomes O29-1/4 using the traditional measurement method. This difference can become important when using Atlas-O track to create a table top plan from a book of tin-plate rack plans.

This is especially true if the plan is for old Lionel 'Super-O' track (O36). Atlas-O O36 is actually O38-1/4 using the tin-plate measurement method. For plans built on a 4'x8' table this means there is 1-1/8" less space between the table edge and track ties. So there may be problems trying to install any operating accessories between the table edge and track shown on the plan.

-- Len Head Rust Scraper KL&B Eastern Lines RR Museum

No, I think you'll find O27 is actually about 12 1/2" radius. Lionel measured the diameter of the track circle across the outside ends of the ties (apparently because this would be the physical space occupied by the circle of track), whereas track radius is usually measured to the track centerline. For example, in their 1950 catalog, Lionel states, "Compete circle of "O27" (8 sections) measures 25" between center rails, 27" overall." But this is not exact either. Lionel states that one #1013 O27 curved track section is 9 1/2" long, which would imply a diameter 24.2" if one got all the joints snug. But if you measure the rails on a 1013, none of them are 9 1/2" long: Running a tape along the outside of the center rail comes out more like 9 3/4" and about 9 5/8" for the centerline length, for a diameter of about 24 1/5". So at best one can say the radius of an O27 circle is approximately 12 1/2". (And the same applies to O31 - stated as 29" between center rails, but NOT to O72, which the 1950 catalog states is 72" between center rails and 74 1/4" overall!) GQ

The larger the number, the wider the curve.

You are correct, Len. Thanks, I still learn something every single day.

Don Cardiff Model Railroad Design Kaneville, IL