What is the difference between scale and gauge?

[Bucksco]

The terms "scale" and "gauge" are words you'll hear often in model railroading. While the terms are closely related, they're often used interchangeably (and incorrectly). The following is a brief explanation of the terms as they apply to model railroading.

Scale

Simply put, scale is the size of a model in proportion to the prototype. HO scale models, for example, have a proportionate size of 1:87. This means that HO scale models are 87 times smaller than the real thing. A list of common scales and their respective proportions follows.

HO = 1:87
OO = 1:76.2
N = 1:160
O, On30 = 1:48
S = 1:64
G = 1:22.5
Fn3 = 1:20.3
Z = 1:220

More than 100 years ago, real railroads set standards for the width of track to allow the exchange of cars between railroad companies and to accomodate cross-country travel. This width, measured between the railheads, is called "gauge." In North America, the standard track gauge of real railroads is 4 feet, 8.5 inches. Therefore, based on the previous scale example, a standard gauge track in HO scale would be 87 times smaller than the real thing.

Narrow Gauge Track

Narrow gauge track is exactly what it sounds like. The railheads are closer together than standard gauge track. In the real world, narrow gauge track is typically used for industrial applications and in areas where conditions prohibit the construction and operation
of standard gauge railroads. You'll frequently find narrow gauge track used in logging and mining operations. Narrow gauge model railroads are indicated by a small letter "n" followed by numerals, which are included with particular scale designation. On30, for example, indicates an O scale model running on narrow gauge track that is a scale 30 inches wide (instead of the standard 4´, 8 "). Similarly, On3 track is a scale 3 feet wide.