Scale and Gauge: The Model Railway Couple


You’ve been to an exhibition, or museum or model village and have caught the Model Railway bug, you go to your local store and are immediately overwhelmed by the variety of shapes, sizes, scales and gauge on offer. But what do these even mean? And why not just have one size to fit all? In this article we aim to give a good grounding in what they are and why they exist.

So what are scale and gauge?

It is important to know the difference between scale and gauge as they are often used incorrectly.

Scale - The scale is the actual size of the model compared to the full size counterpart, so a 1:87th scale model of the Flying Scotsman would be 1/87th the size of the real thing. The larger the number in the scale the smaller the models become. You can also see scale written as a measurement per foot, so you might see 3mm/ft which means three millimetre per foot of length in the original, So a 3cm long model would be based on a 10 foot long locomotive. Picking a standard scale for your collection will mean that all of your models will look right when displayed together.

Gauge- Strictly speaking the gauge is the distance between the running rails on the track and is measured in millimetres. Track comes in standard gauges and model trains are sold to fit these standard gauges.

You will often see them listed by a letter-number designation (O, HO, N, 009 etc). These are actually combinations of both, so models designated HO have a gauge of 16.55 and a scale of 1:87. To make things confusing people will often say that they model in N gauge or HO scale, whilst not strictly correct, this is a common usage for these words.

Scales and gauges are standardised by many competing societies and organisations so you can find that some designations are specific to countries, a model in the UK rated OO will be the same scale but a different gauge to an American OO for example.

So why not just have one?

When the first model railways were released they were designed as toy and every manufacturer built to whatever size suited them. As they became more popular people started to demand that sets became interchangeable and so the first standardised gauges were introduced to enable this. However at this time the ability for the train to run was considered more important than realism, and so the scale of the models was still largely different from between manufacturers, even though the wheel and track gauges had become standardised. The technology of the age also impacted on this. In the UK the trains are smaller than elsewhere in the world and when scaled down to fit the then standard 16.5mm gauge track, the models were too small to fit the motors in, and so they were made bigger to accommodate. This became OO scale which is the most popular in the UK. Whilst modern motors can now fit, being much smaller than they were, OO scale was here to stay and 16.5mm gauge at correct scale is known as HO in the UK.

Eventually these manufacturer sizes became the base sizes we know today, despite being not true to scale they tend to run reliably. In the UK there are two accurate scale to gauge sizes available everywhere, HO and O. This is the main reason for that there are so many variant scale/gauge combinations, there are many scales for every gauge size.

OO Gauge

This is the most popular system for model railways in the UK but not widely accepted elsewhere. As I said before it uses a 16mm gauge rail, but is modelled at 1:76 scale, so it is not an entirely accurate representation. This does however mean that it shares track size with HO scale which helps keep prices down and availability high. It also benefits from having the largest supply of ready to run locomotives, with large companies such as Hornby and Bachmann producing in this size. The Double O Gauge Association promote this scale in the UK.

HO Gauge

Where OO is dominant in the UK, HO is on top everywhere else. Using the 16.5mm track it shares with OO gauge, the scale is smaller at 1:87 which is correct for this track size. However, due to this scale usually modelling trains from outside the UK, particularly America where the original trains are much bigger, there is often very little visual difference in size between the two. Again you can easily find trains and track ready to roll in this size, although they are usually representing European or American originals. Popular brands include Electrotren, Lima, Jouef and Rivarossi

O Gauge

This is the first of the common sizes where international differences become an issue, standard O gauge track is 32mm however a UK O gauge model is scaled at 1:43, in Japan and Russia they are 1:45 scale and in America they are 1:48. This means that if you buy trains internationally, whilst they will all run on your track, they will look odd sizes when displayed next to each other. As this is the largest scale commonly found it requires a lot of space, which it most likely the cause of its decline in popularity in recent years.

N Gauge

This is the second most popular size in the world and the most popular of the smaller sizes, but has different meanings depending on where you are. Whilst all use 9mm gauge track, In Europe and the US the models are 1:160 scale and in the UK they are 1:148 scale. Again this is due to the motors of the time being too large to fit into 1:160 scale versions of the smaller UK trains. Models are available ready to run from Bachmann in the UK and Arnold in the US.

Oh so it’s simple then?

Once you understand what scale and gauge mean, and that the words are not always used correctly, it is much easier to ensure you buy models which work well together. Just pick a size that works for you from the letter scale, and then you can know that anything with the same gauge and scale will work well together. Even the more complicated scales found in narrow gauge or finescale modelling such as HOe and Proto:87, can be broken down into these two questions, What is the gauge? and what is the scale?

Christopher Williams

Back to Articles and Guides