Crain's
Scale and Gauge Encyclopedia
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.