Prototype railroads use the term “gauge” to describe the distance between the rails. Most North American railroads use a “Standard Gauge” of 4ft 8 ½” gauge. Other narrow gauges include 3-foot and 2-foot gauge, though they are more uncommon. Historically, there were several different gauges used in the United States, and most were converted to “Standard Gauge” after the Civil War. Modeling these different gauges can open up different types of modeling, equipment, scenery and time periods. Additional prototype track gauge information is available from Trains Magazine.
The National Model Railroad Association provides Standards and Recommended Practices for the model railroad hobby. NMRA S-1.2 Standards for Scale Models defines different scales and track gauges.
Like the prototypes, there are many different track gauges in OS2R. Traditional O and Proto:48 tackle the “Standard Gauge” railroads. Narrow gauges (On3, On30, On2 and on18) are abbreviated with an “n” for “narrow” and a number to indicate the distance between the rails. For example, if you are modeling three-foot gauge trains in O Scale, the abbreviation would be On3 (O for O Scale, “n” for narrow and 3 for three-foot gauge). Many modelers find that the 1:48 scale proportion is perfect to balance size and detail in these narrower gauges.
Below is an image of the track gauges most commonly modeled in OS2R. Beyond that is a description of each gauge. There is plenty of information on each track gauge and explore the different gauges you can model!
Traditional O (5-Foot Gauge) is the most common form of OS2R out there. It was the first commercially available 1:48 track gauge at 1 ¼” wide, and allows modelers the ability to model standard gauge locomotives and rollingstock. Traditional O track is available in rail codes 148, 125 and 100 rail. Turnouts may or may not have similar options, but are available from smaller suppliers. Most commercially available O Scale cars and locomotives are made to run on 1 1/4″ track. In 1:48 scale proportions, the 1 ¼” track gauge actually measures out to a scale 5′ rather than the prototypical 4′ 8 ½” “Standard Gauge”. This came about because early toy train manufacturers used the 1-1/4″ track gauge (which was invented by Marklin in the 1890s) since it was a nice round number in imperial units, and the small gauge discrepancy was not noticeable. Today, Traditional O remains the most popular part of the OS2R hobby. Modelers who model in Traditional O refer to the gauge as “O Scale 2-Rail.”
Proto:48 (Fine Scale), referred to as P:48, is the name for 1:48 models using scale-correct 4’ 8½” track gauge. The track gauge measures out to 1.177” (NMRA Standards for Proto Models) instead of 1 ¼”. The key differences between Proto:48 and Traditional O comes down to the track gauge and wheelset standards. Proto:48 track is available in rail codes, 125 and 100 rail. Turnouts may or may not have similar options, but are available from smaller suppliers. Most Proto:48 track is hand-laid. Wheelsets have thinner flanges and narrower tire treads that look more prototypical. Most commercially available O Scale cars and locomotives are made to Traditional O track gauge standards. Products and services are available to convert these to Proto:48 standards, with some importers and manufacturers specializing in Proto:48 products.
On3 (3-Foot Gauge) is defined as O Scale locomotives and rolling stock that uses ¾” gauge track (3-feet in 1:48). On3 was one of the first NMRA Proto-Scale standards (referred to as Proto:48n3 in the Proto-Standards; see NMRA Standards). Structures, vehicles and figures are compatible with 1:48 scale proportioned models. A number of prototypes are modeled across the United States, most notably the East Broad Top and Denver and Rio Grande Western.
On30 (30-inch Gauge) started as early as the 1950’s with O Scalers making O Scale 
models using HO Scale wheelsets and track. The HO Scale track gauge (0.65”) measures out to 30 inches in O Scale. Prototypes using this gauge of track vary, as these railroads tended to be used in logging applications, with track that was made easily movable for reuse. Modelers who model in On30 may model freelance roads, or choose to model a prototype 3ft or 2ft railroad with commercially available “Ready-to-Run” (RTR) products.
On2 (2-Foot Gauge) prototypes often follow the Maine 2-foot narrow gauge railroads in the US. Modelers often scratch build locomotives and rolling stock that is specific to those prototypes, with some commercial imports and kits available.
On18 (18” Gauge)
On18 represents 18” gauge railroads by using 9mm track and N scale or HOn30/HOe mechanisms for power. This narrow gauge would have been used mainly in industrial applications where heavy small loads need to be moved. Examples could be a mine, brick maker, a forge, a brewery/winery/vegetable oil producer, small scrapyard, etc. This gauge can be used in conjunction with On3, On30, or O standard gauge as a “feeder” line to bring raw or processed materials to be loaded onto the larger trains for transport to the outside world.