O Gauge trains have a lot to offer no matter which part of the hobby you enjoy most. But for those who are new to model railroading, the various fragments of this gauge can be confusing.
One of the first things you'll notice about most O Gauge trains is that they run on the track with three rails. This system originated early in the 20th Century as a way to eliminate the problems of short circuits with reverse loops.
These trains operate on AC power. The center rail is AC Hot and both outer rails are Ground. Two wires connect the track to the transformer just as with 2 rail trains (the outer rails are connected by a metal bus on the track sections themselves.)
Three Rails vs. Two
So what is the difference between 3-Rail O and 2-Rail? Well, aside from the third rail, today not much!
With the current variety of products, from track to trains to control systems, offered by the major O Gauge manufacturers along with conversion parts available from other makers, there is no reason you can't have any product in either form. There are still some historical trends which tend to typify a 2 vs. 3 rail layout, but the lines are much more blurred than in the past.
For much of the 20th Century, 3-rail trains were marketed mainly as toys and entry-level sets. Combined with the ease of wiring that came with the third rail, many of these train sets featured extremely sharp curves and often the trains themselves were reduced in proportions. This is one reason we refer to it most commonly as O Gauge and not O Scale.
Modelers looking for more realism were drawn to larger, scale models and of course, two-rail track. This aspect of the hobby was filled with craftsman kits, broad curves and even hand-laid track and scratch building.
By the end of the century, however, 3-Rail trains were being made with increased realism and scale-proportions. These are often described as "scale" as opposed to "traditional" models. Many modelers began building 3-rail layouts with large-radius curves and realistic scenery. Some call this "hi-rail" or "3-Rail Scale." Two-railers have benefited as well with an improved selection of products which, with some work, can be run on two-rail layouts.
Converting to 2-Rail
Most 3-rail equipment can be converted to 2-rail relatively easily. For freight cars, wheelsets must be replaced with ones with an insulated axle. Most 2-railers prefer wheels with smaller flanges as well. Most 2-rail trains also use smaller couplers.
Locomotives require a little more work. Not only must the wheels be isolated but a new set of electrical contacts must be made and the third rail pick up eliminated. Many locomotives feature other compromises, like locomotive pilots which turn with the trucks, to accommodate tight curves. Most two-rail modelers correct these changes as well, but there is no reason that two rail and tight curves can't go together.
Control systems, be it conventional control, DCC, or one of the 3-rail manufacturers' command control systems all work with 2-rail trains.
Since there is still much more available for 3-rail than 2, there is little need to convert the other way - but it could be done.
Just in case the differences between 3 and 2 rail weren't enough for O Gauge, there is one more subset which merits attention. You'll notice that in the smaller scales, "gauge" and "scale" are often used interchangeably without a problem. In strictest terms, this can't be done with O.
Although most O Gauge trains are proportioned (scaled) to 1:48, at these proportions the distance between the rails (gauge) is a scale 5 feet. This is slightly wider than the American and European standard gauge of 4 feet 8.5 inches.
Although the distance is only roughly 1/16 of an inch, there are those who strive to correct this discrepancy. Doing this requires re-gauging wheels and in most cases, hand-laying track.