Crain's
Scale and Gauge Encyclopedia
Rocky Mountain House, Alberta, Canada, T4T
2A2
Phone/Fax: 403-845-2527 email us
Updated 10 Jan 2005 c.1998 - 2008 E. R. Crain, P.Eng. All Rights
Reserved
SCALE
AND GAUGE STANDARDS
Railway Pages Index
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.
Click
here to see NMRA Standards web page.
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:
| 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" Scl |
1.766 |
44.9 |
32 |
0.75 |
0.706 |
Ga.
1 |
| 8:
1/2" Scl |
2.5 |
63.5 |
24 |
0.667 |
0.714 |
Ga.
3 |
| 9:
3/4" Scl |
3.5 |
88.9 |
16 |
0.75 |
0.737 |
Ga.
4 |
| 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 the 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
30 to 32 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).
In
Britain and Europe, additional standard scales are defined as:
| |
Model
Track Gauge |
|
Relative
Size |
Comparable
North American |
| Scale
Name |
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.
The
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.
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.
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.
The
narrow gauges for which NMRA provides standards are:
| |
|
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
| 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 |
|
24 |
| 19 |
#1n3 |
1.126 |
|
24 |
| 20 |
#1n2 |
0.752 |
|
24 |
| 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.
Many
model track gauges were chosen arbitrarily to be easy to use and
remember, either in inches or millimeters. Thus the chosen model
gauge may not match the true gauge. The percent error is shown
in the tables for all cases. Some early models adopted track gauges
that were 10 to 15 percent too wide or too narrow for the scale,
making some models look clumsy and out of proportion. Most modern
models are within 5% of the correct track gauge.
As
mentioned above, due to the 3/4 RULE, a standard track gauge from
one scale can be used to represent another gauge in a different
scale. This gives rise to an almost infinite variety of scale
and gauge combinations, some using nearly exact standards and
others using "near-enough" track and wheel standards
from a different scale. Where such inter-scale swapping is (or
could be) done, they are listed in the tables.
For
example, Z scale (standard gauge) track dimensions adequately
represent 30 inch narrow gauge track in HO scale. Note that it
is only the dimensions of the track gauge that match. The size
and spacing of the ties on the track would probably need to be
changed to reflect actual dimensions in HO scale.
Where
the 3/4 RULE holds, such as for locomotive bodies and rolling
stock, some models survive inter-scale mixing with few problems.
In more extreme cases, the expensive and difficult to build locomotive
drive mechanism can be used with a scratch built body. Trucks,
couplers, or track components also may be useful.
The
tables in this book were created on a spreadsheet, using these
conversion factors:
1
inch = 25.4 mm exactly |
1
meter = 3.281 feet exactly |
Ratio
= 12 / (inches/foot) |
inches/foot
= 12 / Ratio |
Ratio
= 12 / (mm/foot) / 25.4 |
mm/foot
= 12 * 25.4 / Ratio |
Ratio
= 1000 / (mm/meter) |
mm/meter
= 1000 / Ratio |
Model
track gauge (inches) = Prototype gauge (inches) / Ratio
|
Model
track gauge (mm) = 25.4 * Prototype (inches) / Ratio |
%
Error To Prototype = ((Model track gauge * Ratio / Prototype
gauge) - 1) * 100 |
(+)
= model tracks are too wide, |
(-)
= model tracks too narrow. |
In
the tables that follow, 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.
ABOUT
THE AUTHOR E.
R. (Ross) Crain, P.Eng. is a Consulting Petrophysicist and a Professional
Engineer with over 40 years of experience in reservoir description,
petrophysical analysis, and management. He has been a specialist
in the integration of well log analysis and petrophysics with
geophysical, geological, engineering, and simulation phases of
oil and gas exploration and exploitation, with widespread Canadian
and Overseas experience.
"I
am a life-long model railroader and have modeled in O27, HO, HOn3,
and N Scales. Failing eyesight brought me to G Scale. My father
started me in model railroading as a tiny tot in 1944 - he scratch
built his first locomotive in 1940, the year I was born, and I
still have this loco on my mantle-piece. I am a Life Member (#517)
of NMRA, a member of the Rocky Mountain Garden Railroaders (Calgary,
Alberta), and have toured a lot of model railways, railway shows,
and garden railways. I have never seen a model railway I didn’t
like. An extensive library of railway magazines and books, covering
topics that appeal to me, sit behind my office desk, ready to
be put to use at a moments notice. I hope these pages can communicate
to you some of my accumulated experience, my successes and failures,
and my love of model railways." |