A Quick Primer on North American Model Railroading Gauge and Scales
Generally North American, European, Great Britain, Japan, and Australia railroad modeling communities each maintain their own standards. For now let’s focus on the Standards applicable to the North American community.
Gauge is the distance between the rails the train runs on. Basically, stability increases with gauge. Narrow gauges are cheaper and faster to build than Broad gauges, while the Broad gauges are able to handle heavier and faster traffic. In reality there are many different regional gauges in use around the world, but for practicality the North American modeling community recognizes 5 gauges:
Universal Standard Gauge (1435.1 mm)
3 1/2 ft/Meter (1066.8 mm – 1000.0 mm)
3 ft (914.4 mm), 2 1/2 ft (914.0 mm)
2 1/2 ft (762.0 mm)
2 ft (609.6 mm).
Scale is the ratio of the model dimension to the prototype. The National Model Railroad Association (NMRA) standards use letters to differential the scales between 1:20.3 through 1:220 for the North America model railroading community.
F indicates a 1:20.32 scale to the prototype
H indicates a 1:24 scale to the prototype
O indicates a 1:48 scale to the prototype
S indicates a 1:64 scale to the prototype
HO indicates a 1:87.1 scale to the prototype
TT indicates a 1:160 scale to the prototype
N indicates a 1:220 scale to the prototype
Since there is effectively 5 possible prototype gauges, narrow gauges are designated with a n42, n3, n30, or n2 subscript. For example; O refers to a 1:48 scale model with a Universal Standard Gauge prototype gauge, while On30 refers to a 1:48 scale model with a 3 ft prototype gauge.
No subscript indicates a Universal Standard Gauge Prototype gauge
n42 indicated 3 1/2 ft or Meter Prototype gauge
n3 indicated 3 ft Prototype gauge
n30 indicated 2 1/2 ft Prototype gauge
n2 indicated 2 ft Prototype gauge