Crain's Scale and Gauge Encyclopedia
and Gauge Standards
PROTOTYPE is a word used to mean the original, full size item that is to be modeled.
SCALE or SCALE RATIO is the ratio in size between an original and a model of the original. A very popular scale ratio for model trains and model cars is 1:87, which translates to 3.5 millimeters equals 1 foot. In the case of HO scale models, they are 1/87th the size of the prototype.
GAUGE or TRACK GAUGE is the distance between the rails of real or modeled railway tracks. The standard track gauge on most North American railways is 56.5 inches, but many other gauges exist. In the illustration at right, "G" is the TRACK Gauge. The drawing applies to real railways as well as model railways. In a few very old books, "G" was measured to the centerline of each rail, but this is no longer done.
SCALE/GAUGE COMBINATION is a track gauge used with a particular model scale. The same gauge of model track can be used in several scales to represent different gauges in these various scales. For example, 1-3/4 inch (45 mm) gauge track is used to portray many gauges in many scales.
The purpose of standards, of course, is to assist in creating at least some minimum compatibility between models, of nominally the same scale, made by different manufacturers. Unfortunately, standardization has not been achieved in any of the larger scales, and has been fragmented in the smaller scales, by separating fine scale and high rail standards from normal, or coarse scale, standards.
There are at least 60 scales in use today; the most common 10 basic scales are listed below:
Note that most scales are approximately 3/4, or 0.750, times the next scale in the list. I call this happy coincidence "CRAIN'S 3/4 RULE".
The 3/4 RULE makes it easy to use track and wheel sets from a smaller scale as narrow gauge components in a larger scale, because 3 foot and meter gauge railway equipment is usually constructed to be about 3/4 the size of standard gauge equipment. For example, the average older standard gauge boxcar is 10 feet wide and 40 feet long. Many 3 foot gauge boxcars are 7 to 8 feet wide and 28 to 30 feet long.
In the USA, the NMRA has traditionally recognized three additional scales, namely OO Scale (1:76.2), O17 Scale (1:45.2), and G Scale (1:22.5). These scales are close to HO, O, and 1/2" respectively. Very recently, NMRA has proposed some additional scales as standards. The additions to the NMRA list are 1-1/2 inch Scale (1:8), M Scale (1:13.5), F Scale (1:20.3), and A Scale (1:29). They were never adopted as standards, which demonstrates a serious lack of respect for the larger scales, and has driven most practitioners to other Associations (GMRA.org and others)
In Britain and Europe, additional standard scales are defined as:
Most of these are considered fine scales, but normal or coarse scale versions also exist. The 3/4 RULE is not as neat for continental scales. Some North American scales. such as Z, N, HO, O, and G Scales, are also common in Britain and Europe, as are a number of lesser used scales not listed above.
STANDARD GAUGE for North America, Britain, and parts of Europe,
Asia, Africa, and Australia is 4 feet 8-1/2 inches (1435 mm).
This strange dimension may go back to Greek and Roman chariots,
which were designed to fit a standard stone road or bridge. Many
early steam locomotives in Britain were made to the same size.
In mountainous regions, and on construction or mine sites, standard gauge was too expensive, so NARROW GAUGE railways were built, often 24, 30. 36 or 42 inches (or equivalent metric gauges).
Although the plural of foot is obviously feet, the gauges are traditionally named with the word foot, or centimeter, not pluralized. Note that some European standard gauge is 1440 mm instead of 1435 mm.
To model one of these track gauges, we would take the original TRACK GAUGE and divide by the SCALE RATIO. For example, at 1:87 scale, standard gauge is 56.5 inches divided by 87, which equals 0.649 inches. As it happens, this is the current NMRA standard for this scale and gauge combination. A 3 foot narrow gauge in 1:87 scale would be 0.413 inches and at 1:22.5 it would be 1.600 inches. The first is an NMRA standard, the second is not.
narrow gauge sTANDARDS
exist for some scales. Names of these scale/gauge combinations
vary between Europe and North America. In Europe, a lower case
letter is appended to the generally accepted scale name, for
example HOm, where the "m" stands for meter gauge. The other
letters used are "e" for 800+/- mm gauge (30"+/-), "i" for
industrial 650+/- mm gauge (24"+/-), "f" for field railways 450
mm+/- gauge (15 to 18"), and "p" for park trains 300+/-mm gauge
(12"+/-). Hence scale/gauge names like HOe, Gm, and Of can be
found in advertizements for commercially available models. Just
which actual gauge track is required is not usually specified.
Whether any or all of these proposals are adopted or modified, only time will tell.
NMRA Standard S-1 and other NMRA documents incorrectly refer to theae narrow gauge names as SCALES, instead of GAUGES. Additional standards are specified in NMRA recommended practices for fine scale and high rail versions of some gauges.
In the tables, track gauge dimensions are given in inches and millimeters. Where two dimensions are given, these are the minimum and maximum dimensions given in NMRA Standards or Recommended Practices (either official or proposed). Where a single value is shown, a non-NMRA source was used.