Actual diameter of fixed radius curved track from different commercial manufacturers

Lionel O-72 was made to a 35.88" center rail radius, just about 72" diameter, a little different than the rest of the Lionel tubular world(14.14" O Gauge, 12.37" O-27, etc.). MTH kept the 14.14" center rail radius for their "O-31", but when Lionel released their FasTrack "O-31", it was with a 15.5" radius. There really is no uniformity among manufacturers, except that FasTrack is all made to a center rail radius measured system. Even GarGraves varies in measurement between wood tie & plastic tie track.

ADCX Rob posted:

 Even GarGraves varies in measurement between wood tie & plastic tie track.

Believe that is due to the differing number of ties (and corresponding track length) between the plastic and wood versions.

Typically the variations are small, and nominal 072, for example, is nominal 072, regardless of source. Any variations will tend to be lost in the vagaries of actually laying the track in real-world (layout) conditions and, ah, workmanship (as in mine). A long, sweeping, highly visible curve is best done with pre-curved sections of the same brand for obvious reasons.

I have heard/read that some 3-rail manufacturers have measured diameter from the outside running rail at times, rather than the center, conductor rail. True or not, I do not know.

Lionel has always measured their tubular track from the outside edge of the ties.  This illustration from their 1954 manual also appeared in their 1965 manual.

While I don't know about 072 track or curves, I can tell you that there are different sizes in curves among the 042 size curves. Lionel & K-Line are very close to matching in size, while Gargraves will be almost half an inch larger circle then Lionel's 042. Also Gargraves and Ross have different sizes for the 042 curves in their switches, Gargraves is a little larger in size then Ross.

Not sure on this but I have heard that Atlas O makes an 045 curve.

Lee Fritz

Yes, Atlas make O45.  They start at O27 and go up, adding 9 inches to the next turn.  This gives a 4.5" center to center spacing.  And Atlas, as well as FasTrack, measure to center rail.