I'm working on an O Scale layout and am confused by the nomenclature Atlas
uses, they label the track as 'O-72 Full Curve', 'O-54', 'O-27', etc.
Is this Radius or Diameter? I believe it's diameter because if you draw a
full circle in RTS of the O72 it is 72" in diameter.
Yet their HO scale is specific to Radius.
Just want to get this clear, can any of you O Scalers help me out here?
Bearridge RailRoad (BRR)
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!
Model Railroad Design
an "O72 Radius"
an O in front of
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.
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