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Guide to Understanding Scales and Gauges used in Model Railways

scale and gauges


Lots of people get confused between the different sizes of models available, and with the different scales and gauges when modelling model railways. Hopefully, this guide will make things clearer.

In model railways, "scale" and "gauge" are often used interchangeably, but they refer to different aspects of the models.

Gauge: Gauge refers to the distance between the rails on the track. It is typically measured in millimetres or inches. The gauge determines the width of the track and thus the compatibility between trains and track components.

Scale: Scale refers to the ratio of the model to the real-life object it represents. It is typically expressed as a ratio. For example, in OO scale, 1 unit of measurement on the model represents 76 units of the same measurement in real life. The scale determines the size of the trains and other elements in the model, such as buildings, people, and scenery.

So, in summary:

Scale determines the model size relative to the real thing.

Gauge determines the width between the rails on the track.

While they are related, they are not the same thing and both are important considerations when building a model railway layout.

Common UK Gauges

In the UK, several railway gauges have been used historically and are still used today. Here is a detailed description of the main ones:

Standard Gauge

Width: 4 feet 8 1/2 inches (1,435 mm)

Usage: This is the most common gauge used in the UK and globally. It became the standard gauge for British railways largely due to the influence of George Stephenson, who used it for the Stockton and Darlington Railway and the Liverpool and Manchester Railway.

Significance: The adoption of standard gauge facilitated interoperability and network expansion across the country and internationally.

Broad Gauge

Width: 7 feet 1/4 inch (2,140 mm)

Usage: Developed by Isambard Kingdom Brunel for the Great Western Railway (GWR).

Significance: Broad gauge was intended to provide greater stability and speed for trains. However, it led to incompatibility issues with the standard gauge network.

Outcome: Broad gauge was gradually phased out in favour of standard gauge by 1892 after the "Gauge Act" of 1846 mandated standard gauge for new railways.

Narrow Gauge

Various narrow gauges have been used primarily for industrial, rural, and light railways. Some of the notable ones include:

3 feet (914 mm)

Usage: This gauge was used in some early industrial railways and tramways.

2 feet 6 inches (762 mm)

Usage: Used for some light railways and industrial lines. Examples include the Leek and Manifold Valley Light Railway.

2 feet (610 mm)

Usage: Commonly used for industrial, mining, and some rural railways. Notable examples include the Ffestiniog Railway and the Lynton and Barnstaple Railway.

15 inches (381 mm)

Usage: This is an even narrower gauge, used mainly for miniature railways and some amusement park rides. An example is the Romney, Hythe & Dymchurch Railway.

Irish Gauge

Width: 5 feet 3 inches (1,600 mm)

Usage: Though primarily used in Ireland, this gauge has been used in some parts of Northern Ireland and for the Manchester and Milford Railway.

Significance: This gauge is standard for Irish railways but not widely used in mainland Britain.

Tramway Gauges

Several gauges have been used for tramways, typically narrower than the standard gauge. Common tram gauges include:

Standard Gauge (4 feet 8 1/2 inches): Used in many British tramways to allow interoperability with mainline railways.

3 feet 6 inches (1,067 mm): Used in some tram systems like the Isle of Man.

Historical Context

The choice of gauge was influenced by various factors, including geography, economic considerations, and the engineering philosophies of influential figures like George Stephenson and Isambard Kingdom Brunel. The "Gauge War" between the standard and broad gauge advocates was a significant period in British railway history, ultimately resolved by the adoption of standard gauge.

In summary, the UK has seen a variety of railway gauges, each serving different needs and regions. The standard gauge is now predominant, providing a unified and efficient railway network.

What are the differences between the scales

In the UK model railway community, various scales and track gauges are used to cater to different preferences and constraints. The main scales include OO (1:76), N (1:148), and O (1:43.5), with corresponding track gauges of 16.5 mm for OO, 9 mm for N, and 32 mm for O. Each scale and gauge combination has distinct advantages and disadvantages.

We have intentionally not discussed the advantages and disadvantages of TT scale. Notably, TT scale encompasses two different scales running on the same track gauge, with the newer TT:120 being more accurate. As TT:120 is relatively new, it would be premature to list its advantages and disadvantages due to the currently limited range of available products. We will provide a detailed analysis once the scale becomes more established and more products are available.

OO Gauge (1:76 Scale, 16.5 mm Gauge)

Popularity: OO is the most popular scale in the UK, offering a vast range of models, kits and accessories, making it easy to build a comprehensive layout.

Detail vs. Space: Strikes a balance between detail and space requirements, allowing for reasonably detailed models without needing an excessive amount of space.

N Gauge (1:148 Scale, 9 mm Gauge)

Space Efficiency: Ideal for hobbyists with limited space, as the smaller scale allows for more complex and extensive layouts in a compact area.

Portability: Easier to transport and store, making it suitable for those who need to move their layouts frequently.

O Gauge (1:43.5 Scale, 32 mm Gauge)

Detail and Realism: Larger scale allows for highly detailed and realistic models, appealing to enthusiasts focused on precision and visual impact.

Robustness: Typically more durable, making it suitable for hands-on operations and displays.

OO Gauge

Space Requirement: While more compact than O gauge, OO layouts still require a significant amount of space, which can be a constraint for some hobbyists.

Scale Inaccuracy: The gauge (16.5 mm) is slightly narrow for the scale (1:76), which can affect realism for purists.

N Gauge

Detail Limitation: The smaller scale limits the level of detail that can be realistically achieved on models, which might not satisfy enthusiasts seeking high precision.

Handling Difficulty: Small size can make handling and maintaining models more challenging, particularly for those with dexterity issues.

O Gauge

Space and Cost: Requires substantial space and is generally more expensive, both in terms of models and layout materials, making it less accessible for beginners or those with budget constraints.

Portability: Larger and heavier, making transportation and storage more difficult compared to smaller scales.

In summary, the choice of scale and gauge in UK model railways involves a trade-off between space, detail, cost, and ease of handling. OO gauge offers a balanced approach with widespread availability, N gauge excels in space efficiency, and O gauge provides superior detail and durability, catering to diverse preferences within the hobbyist community.

Model railway scales

The more popular scales and gauges are discussed below. This is not a definitive list as there are many more, but these are the most common scale and gauge combinations used in modelling in the UK and represent a wide range of model sizes.

Small Scales

T Gauge
0.61mm:1ft, 1:450 scale
3mm track gauge (inside track width)
This is a commercial gauge produced and used mainly in Japan.

Z Gauge
1.5mm to 1ft, 1:220 scale
6.5mm track gauge (inside track width)
This is a commercial gauge produced by Marklin in Germany. It is relatively expensive and specialist.

2mm Scales

N Gauge
2mm to 1ft, 1:148 scale in UK
9mm track gauge (inside track width)
This scale is half the size of 00 and therefore you should in theory be able to lay four times as much track. This is becoming very popular with a full range of locomotives, rolling stock and accessories available from many companies.

2mm Scale
2mm to 1ft, 1:152.3 scale
9.42mm track gauge (inside track width)
The purists of N gauge developed this scale. It is slightly more accurate than the mainstream scale of 1:148.

3mm Scales

TT120 Gauge
2.54mm to 1ft, 1:120 scale
12mm track gauge (inside track width)
A new scale and range of products started by Peco Model Railways and is a more accurate scale for 3mm modellers.

TT Gauge
3mm to 1ft, 1:101.6 scale
12mm track gauge (inside track width)
This gauge originated in the USA and was also produced at 2.5mm to 1ft, 1:120 scale. Enthusiasts using this scale need specialist support through the Three Millimetre Society.

4mm scales

HO Gauge
3.5mm to 1ft, 1:87 scale
16.5mm track gauge (inside track width)
This is the major gauge used outside the UK. At 3.5mm to 1ft, the track gauge at 16.5mm is virtually exact to scale for the standard gauge. When using this gauge it must not be confused with 00 gauge, HO gauge is almost 15% smaller. One can run HO gauge rolling stock on 00 gauge layouts, the track gauges both being 16.5mm, but the difference in scale will immediately become very obvious.

00 Gauge
4mm to 1ft, 1:76 scale
16.5mm track gauge (inside track width)
This is the most popular scale for British modellers and is probably the best supported in the industry with a wide range of ready-to-run models, kits and accessories.. However, the track gauge is considered too narrow to be accurate by many. This makes this scale a bit of a compromise. Smaller track profiles make this incorrect scaling less noticeable, and increasing the sleeper spacings makes it even less noticeable.

EM Gauge
4mm to 1ft, 1:76 scale
18.2mm track gauge (inside track width)
This scale is an attempt to make the 00 layout track gauge more realistic. At 18.2mm it still falls short of the ideal 18.83mm, but was felt by those involved to be near enough.

P4/S4 Gauge
4mm to 1ft, 1:76 scale
18.83mm track gauge (inside track width)
As EM is still slightly under scale, P4 was established. Locomotive and rolling stock kits are available to fit this gauge.

009 Gauge
4mm to 1ft, 1:76 scale
9.00mm track gauge (inside track width)
The most popular narrow gauge scale as this allows for the use of N gauge chassis and most 4mm accessories. Track and turnouts are available from a wide range of manufacturers.

00n3 Gauge
4mm to 1ft, 1:76 scale
12mm track gauge (inside track width)
Used for models of the Isle of Man railways and the Irish 3ft gauge systems.

7mm Scales

0 Gauge
7mm to 1ft, 1:48 scale
32mm track gauge (inside track width)
This scale is commonly found in the USA.

7mm to 1ft, 1:45 scale
32mm track gauge (inside track width)
Scale used mainly in Germany, Japan, Russia, Czech

7mm to 1ft, 1:43.5 scale
32mm track gauge (inside track width)
This scale has become more popular due to the availability of a large range of quality locomotive and rolling stock kits. Technically the inside track width of 32mm is 3% under scale.

Scale Seven
7mm to 1ft, 1:43.5 scale
33mm track gauge (inside track width)
This is the scale track option for 7mm scale with a 33mm width between tracks.

0-16.5 Gauge
7mm to 1ft, 1:43.5 scale
16.5mm track gauge (inside track width)
Uses OO gauge spaced track allowing commercial OO gauge locomotive chassis to be used under narrow gauge 7mm kits.

Larger scales

Gauge 1
10mm to 1ft 1:32 scale
45mm track gauge (inside track width)
This scale is mainly used outside for electric and live steam operations.

Scale Comparison chart

Name Gauge Scale mm/foot Notes
Standard 4ft 8.5ins 1:1 304.8 World Standard
Broad 7ft 1:1 304.8 Brunels Gauge
G 45mm 1:24-8 15 Narrow Gauge 3ft
Gn15 16.5mm 1:24 15 Representing 15ins Gauge
SM45 45mm 1:19 16 Narrow Gauge 3ft
SM32 32mm 1:19 16 Narrow Gauge 2ft
1 45mm 1:30.5 10  
0 32mm 1:43.5 7  
Scale7 33mm 1:43.5 7  
016.5 16.5mm 1:43.5 7 Narrow Gauge 2ft 3ins but used for 2ft 0ins - 2ft 6ins
0n30 16.5 mm 1:48 6..4 USA Narrow Gauge 2ft 6ins
09 9mm 1:43.5 7 Miniature Gauge 15ins
S 7/8ins 1:64 3/16ins  
H0 16.5mm 1:87.1 3.5 Worlds Most Popular
H0n3 10.5mm 1:87 3.5  
00 16.5mm 1:76.2 4 UKs Most Popular
00 Broad Gauge 28mm 1:76.2 4 Brunels Broad Gauge in 00
00n3 12mm 1:76.2 4  
009 9mm 1:76.2 4  
EM 18.2mm 1:76.2 4  
P4 18.83mm 1:76.2 4 Exact Scale 4mm
3mm (TT) 12mm 1:101.6 3  
TT120 12mm 1:120 2.54  
N 9mm 1:148 2  
2mm Finescale 9.42mm 1:152.4 2  
Z 6.5mm 1:220 1.4 Smallest Commercially Available

Thickness chart

We found this handy table that gives the actual dimensions of thicknesses of different sheets sizes. Plastic sheets and extrusions are generally measured in THOU so this conversion chart gives metric and imperial real thicknesses of those sheets.

Material Thickness Prototype Dimensions
Thou Actual Inches Actual mm Inches 2mm Inches 3mm Inches 3.5mm Inches 4mm Inches 7mm
5 0.005 0.127 1 3/4 5/16 1/2 1/4
10 0.010 0.254 1 1/2 1 1/8 7/16 3/4 1/2
15 0.015 0.381 2 1 1/2 1 1/4 1 5/8
20 0.020 0.508 3 2 1/4 1 7/8 1 1/2 3/4
25 0.025 0.635 4 3 2 1/2 2 1
30 0.030 0.762 5 3 5/8 3 2 1/4 1 1/2
40 0.040 1.1016 6 4 1/2 3 3/4 3 1 3/4
50 0.050 1.27 8 6 5 4 2
60 0.060 1.524 9 6 3/4 5 5/8 4 1/2 2 1/2
70 0.070 1.778 11 8 1/4 6 13/16 5 1/2 3
80 0.080 2.03 12 9 7 1/2 6 3 1/2
90 0.090 2.286 14 10 1/2 8 7/8 7 4
100 0.100 2.54 16 12 10 8 4 1/2