O scale originated in Germany around the turn of the 20th Century. Marklin, the German toy manufacturer who originated O scale, chose 1/48th, or "1/4 inch to the foot" scale because they already used it in manufacturing doll houses. The first O scale trains didn't use electricity, they were wind-up toys.
Variety is the Spice of Life
O scale model trains come in three-rail and two-rail varieties. To make things even more confusing, you'll see numbers like O27, O36, and O45 on packaging for locomotives and track pieces. The numbers following the O are curve diameters in inches. Unlike other scales, O scale curves are stated in diameter, not radius.Three-Rail Systems
Three-rail O scale track, as the name implies, has a third rail running down the center of the track. Three-rail systems use 18 volt alternating current to power the trains, with the center rail connecting to one leg of the transformer and the outside rails wired together to the other.In order to make three-rail track look more realistic, several manufacturers have introduced O scale track with black center rails. The black camouflages the middle rail to some extend.
A two rail locomotive or powered car will not run on a three-rail system.
Two-Rail Systems
Two-rail O scale trains work like model railroad trains in other scales; they run on direct current (DC) power where the left rail (in the direction of locomotive travel) is the plus (+) voltage and the right rail is the minus (-). This is, of course, unless they have been configured for Digital Command Control (DCC) which supplies power and control signals using a pulsed alternating current.A three-rail locomotive or powered car will cause a short circuit if placed on a two rail layout.
Reverse Loops
While three-rail O scale track doesn't look as realistic as two-rail track (even with a blackened center rail), it gives three-rail trains the ability to be turned around through a reverse loop and run back the way they came on the same track. A reverse loop wasn't practical in two-rail systems until the recent introduction of DCC and a unit for "auto-reversing" power to the tracks.Standard O vs. O27
Image courtesy of Lionel, LLC.
The original three-rail "Standard" O had a minimum curve diameter of 31 inches. Standard O track was comprised of three tubular rails on insulating railroad ties. Track joiners were pins inserted into the tubes.
Lionel later introduced O27. Some references indicate that O27 was originally introduced for 1/64th scale trains (the same as S scale) with an out-of-scale track gauge. O27 used the same 1.25 inch gauge as Standard O scale, but the minimum curve diameter was 27 inches. Also, Standard O gauge ties and rails had a total height of 11/16ths of an inch while O27 had a combined tie and rail height of 7/16ths of an inch, using smaller tubes and pins.
Lionel later introduced O27. Some references indicate that O27 was originally introduced for 1/64th scale trains (the same as S scale) with an out-of-scale track gauge. O27 used the same 1.25 inch gauge as Standard O scale, but the minimum curve diameter was 27 inches. Also, Standard O gauge ties and rails had a total height of 11/16ths of an inch while O27 had a combined tie and rail height of 7/16ths of an inch, using smaller tubes and pins.
Rectangular Rails
While tubular O scale track is still available, modern O scale tracks, including integrated roadbed track have rectangular rails like those used in real-life railroading and other model railroad scales; rail height is usually Code 100 or Code 125. But the radius of curved sections is expressed in a notation like O27.Getting Around the Curves
O scale curves are available in diameters of O27, O31, O36, O42, O45, O54, O63, O72, O81, O90, and O99. Needless to say, to make a 180 degree turn with O99 curves takes a lot of room. Not all diameters are available from every manufacturer, or in every brand of track.Typical O scale train sets are packaged with O36 curves, usually including straight sections to make an oval of approximately 36 inches by 48 inches.