justify the cost of "O" scale model trains - Model Railroader Magazine
To answer your question, O gauge(notice my lack of the use of the term scale) comes in three basic flavors.
1. Traditional
2. Hi-rail
3. Two-rail scale
Traditional is characterized by 3 rail track, generally less than scale pieces, minimal details, and tight curves. Traditional can further be divided into two categories, Standard O and O27. O27 pieces are generally close to S scale in size(1/64), and run on curves 27" in diameter. Standard O pieces are larger, usually somewhere around 1/53 scale or so(although they sometimes might very close or right on full 1/48 scale), and usually require minimum curves of 31" in diameter. Keep in mind that both Standard O and O27 pieces are often "selectively compressed" which means that they may be full scale in width and height, but shortened lenghtwise. Standard O and O27 can generally be mixed together just fine on the same layout, and even in the same train.
Hi-rail is characterized by more emphasis on scale size, detail, and scenery, although it still uses three rail track. Most hi-rail pieces are full 1/48 scale, have excellent details, and run on 54" and 72" diameter curves(although many pieces are capable of negotiating curves as sharp as 31", given the fact that they look horrible and don't always run well). Some hi-railers run on traditional Lionel-style tubular rails, while most opt for the T shaped rail alternatives from Atlas and Gargraves. Many hi-railers use traditional Lionel-style tinplate knuckle couplers with deep tinplate flanges, although a small but increasing number are switching over to Kadee type couplers with scale flanges. Scale flanges can't be used on tubular track, but Kadee couplers will still mate freely with Lionel couplers. It's also possible to run traditional non-scale equipment on Hi-rail layouts.
Two rail full O scale is a small but increasing segment of the hobby, thanks, in part, to affordable offerings from Atlas O. Many people run DC or DCC engines, although a good many of the new arrivals are using AC with Lionel's Trainmaster Command Control system(TMCC). TMCC allows the easy inclusion of an excellent quality digital sound system, as well as remotely operatable couplers(now available in Kadee style) and smoke units.
