Australian Model Railway Association NSW Inc.

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Scales and Gauges

What do railway modellers mean by these two words? Although they are often used interchangeably, they are not the same.

  • Scale refers to the fraction of the size of the prototype (the real thing) that applies to every single dimension of the model in question. There are an infinite number of scales — as many as there are fractions or ratios — but in model railways the availability of products in specific scales limits the practical choices.

    Note from the table below that models built to HO and OO  scales are different.  So buildings and other structures built to OO scale (usually British) are larger than the same items built to HO scale.  However model locomotives and rolling stock in both scales use the same 16.5 mm gauge track. The difference is small enough for the same commercially available items such as people, trees, structures and vehicles to be used by modellers of both scales.  Locomotives and rolling stock are completely compatible.

  • Gauge is the actual distance between the rails of the track used in the model in question. Just as there have been many different gauges in the real world, the one scale can use many different gauges of track. See the examples in the model below: note that HO scale is 1:87 and N scale is 1:160. But see that HOn2-1/2, which is where a two-and-a-half-foot narrow gauge railway is modelled in HO scale, actually uses the same 9mm gauge track that N scale does. This is because two and a half feet in HO scale measures almost exactly nine millimetres. This happy coincidence means that proprietary N gauge track is useful for modellers of that HO narrow gauge, saving them many hours of track construction.

Of the three scales modelled at AMRA in NSW, O gauge is the oldest and is adopted by those interested more in the locomotives and rollingstock than the scenery through which they run. HO gauge, which is literally "half O", is by far the most popular today because it offers a good balance between space, size and detail. N scale is the newest and is gradually gaining a wider acceptance as more products become available, particularly in New South Wales prototype. As the N gauge layout at AMRA demonstrates, it allows spectacularly epic landscapes to be depicted in a compact area.
Scale Proportion Millimetres to the foot Track gauge
G 1:22.5 13.5 44.5mm
O 1:43.5 7 32mm
On30 1:43.5 7 16.5mm
OO 1:76 4 16.5mm
HO 1:87 3.5 16.5mm
HOn3-1/2 1:87 3.5 12mm
HOn2-1/2 1:87 3.5 9mm
TT 1:120 2.5 12mm
N 1:160 1.9 9mm
Z 1:220 1.4 6.5mm

For further reading, there is a comprehensive article called Which Scale is "Best"? by Steve Chapman from AMRA's Journal of September/October 1990.

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AMRA NSW is dedicated to promoting the hobby of model railways throughout New South Wales.
These pages last updated 16 June, 2017 by Damien George.