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Scale and Gauge Standards

The National Model Railroad Association (NMRA) has established wheel and track standards or recommended practices for some scale and gauge combinations used in North America. Similar organizations in Britain and Europe have also set standards (MOROP and NEMA), some of which conflict with NMRA standards. Some small, and not so small, special interest groups (SIGs) have also set their own standards to permit equipment interchange between modelers who follow these, often more rigourous, standards. You may find minor incompatibility in wheels and track from different countries or different manufacturers.

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.

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. Note that some European standard gauge is 1440 mm instead of 1435 mm.

Standard gauge was adopted by law in the USA in 1886. At that time a census of railways showed 25 different gauges in use across the country. Many railways were built, and some still operate, with wider gauges. One of the widest gauges, 7 feet 1/4 inch (2.14 meters), was used in the early days in Britain. These are usually called WIDE GAUGE or BROAD GAUGE. Some logging railways, especially those with horses for dragging logs, used gauges of from 6 to 9 feet.

There are also NARROW GAUGE railways, with the gauge less than 4 feet 8-1/2 inches, discussed later on this page.

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.
 

MODEL RAILWAY STANDARDS IN NORTH AMERICA
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 and coarse scale standards.

There are at least 60 scales in use today; the most common 10 basic scales used to represent STANDARD GAUGE railways in North American are listed below. The track gauge dimensions and many other detail standards for these scales have been published by NMRA, the first attempts dating from 1936.

Scale Name	Model Track Gauge inches	mm	Scale Ratio 1:xxx	Relative size To Next Scale	Relative Size To Next Gauge	Gauge Name
1. Z Scale	0.257	6.53	220	0.727	0.728	Z
2. N Scale	0.353	8.97	160	0.75	0.749	N
3. TT Scale	0.471	12	120	0.726	0.726	TT
4. HO Scale	0.649	16.5	87.1	0.735	0.742	HO
5. S Scale	0.875	22.2	64	0.75	0.7	S
6. O Scale	1.25	31.8	48	0.667	0.708	O
7. 3/8" Scale	1.766	44.9	32	0.75	0.706	Ga. 1
8. 1/2" Scale	2.5	63.5	24	0.667	0.714	Ga. 3
9. 3/4" Scale	3.5	88.9	16	0.75	0.737	Ga. 4
10. 1" Scale	4.75	121	12	----	----	7.5 inch

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. Recently, NMRA 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 represent the latest trends in Large Scale modeling practice but sadly, they were never adopted as standards, which demonstrates a serious lack of respect for  Large Scale members, and has driven most practitioners to other Associations (GMRA.org and others).

MODEL RAILWAY STANDARDS IN BRITAIN AND EUROPE
In Britain and Europe, additional standard scales are defined by MOROP and NEMA.

Scale Name	Model Track Gauge			Relative Size		Comparable North American
	inches	mm	Scale Ratio 1:xxx	To	Next	Scale	Ratio
1. 2 mm Scale	0.371	9.42	152	0.67	0.79	N	160
2. 3 mm Scale	0.472	12.00	102	0.75	0.63	TT	120
3. 4 mm Scale	0.750	19.10	76.2	0.57	0.60	OO	76.2
4. 7 mm Scale	1.250	31.80	43.5	0.70	0.71	O	48
5. 10 mm Scale	1.766	44.90	30.5	0.71	0.71	3/8"	32
6. 14 mm Scale	2.500	63.50	21.7	0.88	0.71	G	22.6
7. 16 mm Scale	3.500	88.90	19.1	----	----	3/5"	20

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.

MODEL RAILWAY STANDARDS FOR NARROW GAUGE MODELS
In mountainous regions, and on construction or mine sites, standard gauge was too expensive or the equipment was too large to be practical, so NARROW GAUGE railways were built, often 24, 30. 36 or 42 inches (or equivalent metric gauges).

 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.

In North America, the descriptions use the scale name, followed by a lower case "n", and a number. A single digit number represents the gauge in feet, a two digit number represents the gauge in inches, for example On3, Sn3, HOn3, or Nn3, (3 foot gauge) and HOn30 or On30 (30 inch gauge). Stranger comonations such as Gn15 and HOm can be found, as well as HOn2-1/2 (for HOn30).

Crain's Scale/Gauge Encyclopedia tables will show you which track gauge to use to represent these various combinations.

The narrow gauge names and scales for which NMRA provides standards are shown in this table.

		Track Gauge
	Gauge Name	inches	mm	Scale Ratio 1:xxxx
1	Nn3	0.250	6.35	160
2	HOn3	0.413	10.50	87.1
3	HOn2	0.276	7.01	87.1
4	Sn3	0.563	14.30	64
5	On3	0.750	19.10	48
6	On2	0.500	12.70	48

NMRA has proposed adding the following standards but have not followed through.

7	Mn2	1.772	45.0	13.5
8	Fm	1.949	49.5	20.3
9	Fn3	1.772	45.0	20.3
10	Fn30	1.476	37.5	20.3
11	Fn2	1.181	30.0	20.3
12	Gm	1.772	45.0	22.5
13	Gn3	1.614	41.0	22.5
14	Gn30	1.259	32.0	22.5
15	An42	1.476	37.5	29
16	An3	1.259	32.0	29
17	An2	0.886	22.5	29
18	#1m	1.259		32
19	#1n3	1.126		32
20	#1n2	0.752		32
21	TTn3	0.300		120

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.