... 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.
This is one of those "oh no, not this again" questions but it does merit discussion. In addition to all of the above, read through Greenburg's Guide to post war motive power.
In some instances, O27 vs O Gauge were used to signify good vs. bettter in the Lionel product line. O27 engines tended to be smaller and lower in detail and cost than O Gauge. O27 engines were usually numbered with 4 digits (like my 2056 Hudson, 2033 Alcos, 1615 Switcher, etc. and O Gauge engines were 3 digits - but not always. O27 engines could always run on O Gauge track, but O Gauge might not run on O27.
To add to the madness, in some cases, the same darn engine was simply sold as two different items. I think the 646 and 2056 were identical engines, the 646 for O Gauge and 2056 for O27, but no differences in the two.
The famous Santa Fe F3's were normally marketed as O Gauge, but the 2243 had a motor removed (cheapenend) and was marketed as O27 for a period.
The newer term that adds to the confusion is "scale" which simply means precise 1:48 scale throughout. From what I glean, very few products prior to the modern era were truly "scale". At first, I thought "scale" was synonymous with "O Gauge" but quickly learned that was not correct. There are actually a couple scale engines that are small enough to handle 27" curves.
So, as someone pointed out earlier, I think the best use of the O27 and O Gauge terms today is for the track, O27 being lower profile tubular track that's available in 42" and 54" and O Gauge being higher profile that's available in 31", 42", 54", 72", maybe more. O Gauge track, I believe, has better conductivity, but O27 looks closer to scale size.
With engines, I think the terms now are scale vs. semi-scale (although non-scale would be better) have become the point of differentiation.
With rolling stock, here's where you have to be very careful if you're in the non/semi-scale market. Scale is scale across the manufacturers, but they use different definitions and sizings in semi-scale. I have found Lionel, K-Line, and Industrial Rail (now owned by Atlas) rolling stock to be comparable and easily interchanged without looking out of place. MTH's semi-scale rolling stock (Rail King line) tends to be larger than the others. With WBB, they have a nice mix that includes (a) small/non-scale; (b) small/scale; (c) larger/semi-scale; (d) larger/scale. Atlas seems to focus on scale. I don't know about RMT which seems to be growing and worthy of consideration.
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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.