... O27 track is lighter and the curves are sharper, making a 27" diameter circle. Hence the name "O27." ...
O27 refers to the sharpest and most commonly available curves for the "lightweight" version of conventional 3-rail track. However it is also possible to get O42 and O54 curves in the "O27 profile" track, and some older compatible Marx track is "O34".
The sharpest curves in "Standard O" conventional 3-rail track are O31, but O42, O54 and O72 curves are also avaliable.
The two-digit numbers generally refer to the overall diameter of a circle of those track pieces, in inches.
In another post, I discuss use of different curve radii for good effect.
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You original question mentioned collecting, which leans me to think you are looking at train items, perhaps on ebay or somewhere where they list it as "027" or O gauge.
It'll take a bit to grasp the difference, especially when at times there wasn't. Best advise would be to attend a local train show and ask the simple questions. Most train guys are more than willing to help when asked. At just one show, you'll be able to grasped not only the 027/O gauge question, but also the "Scale", HO, S, N, Z and Large Scale.
If you're new to the hobby and unfamiliar with items you're bidding on or planning to buy via mail order or such, NEVER, NEVER, NEVER pay attention to the "O", "O27", "scale" descriptions/designations until AFTER you have thoroughly researched the precise item you're thinking of buying.
I imagine 90% of the people selling this stuff--and especially on eBay and other such sites--have no idea of the meaning--past or present--of these designations.
In the early Lionel catalogs O27 was a marketing term for "cheaper" sets. The engine was often exactly the same as the "O" version, but with different cab numbers: the K4 O27 was 2025, the O was 675. The price difference for the engine and tender alone was 50 cents...$24.50 vs $25.! Set-wise, the O27 came with a stipped-down caboose (no lighting and less details). The Turbine was similar. The Berkshire only came in O. I assume it was too long for O27 curves.
Today many new trains need a curve wider than O31...some MUCH wider. Even O36 is now minimum for many new trains.
My original layout was 027 and I too liked the lower profile of the rails but I could not run O gauge engines until I found the same profile track with 031 curves. I believe that Lionel made this wide curve 027 track back in the Late 70'S /Early 80'S. My current layout uses Gargraves track, Ross and Gargraves switches as it looks more realistic to me and I like to form my own curves.
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I believe the source of source of confusion comes from referring to O and O27 as if they are two different gauges (O gauge vs. O27 gauge). They are actually the exact same gauge - 1-1/4" between the inside edges of the rails. The differences are in the rail height and the traditional minimum diameters (27" outer diameter vs. 31" outer diameter). It would be more appropriate call it O size or 027 size track.
Lionel's O27 switches never appealed to me. Their size, shape, and operation led me to toss about 12 of them a few weeks ago. I offered them on the Buy/Sell board for $3 each, and had no takers, so I'm not alone in disliking them.
This thread motivated me to dig out my O27 track and set up an oval on the floor. I ran a 2037 on that and a 2018 (same loco style) on a loop of FasTrack. What a difference in the noise levels. The O27 track was nearly silent, while the FasTrack was quite loud.
Originally Posted by HOPPY:
I like the lower profile rail of 027 myself. Wish 'L' would expand it to all the
diameters and switches that their O gauge has. Oh well . . .
[quote]Any idea if a Lionel U30C Diesel Engine (6-18276) will work on O-27 curves? No switches are used in the layout. Thanks for your help![\quote]
No, minimum curve is 31 (it's listed in the catalog). Flanges will work on O-27 style track but you need O-31 diameter curves or larger. At 18" long the model is scale or very close. On an O-27 curve the overhang will be really, really, really bad and that's assuming the three axle trucks wont bind on a curve that tight.