In 1899 the Society of Model Engineers recommended five standard model gauges, originally:
The ones that live on today in the UK are Gauge 3, Gauge 1, Gauge 0.
Gauge 3 was the largest of the scales and runs on 2.5” gauge track, 63.5mm, with a scale of 17/32 or 13.5mm/foot, about 1:22.5. Due to its size it is, to all intents and purposes, exclusively a scale for the garden.
Gauge 1 is the most popular of the larger scales running on standard gauge track in the UK and runs on 45mm track to a nominal scale of 10mm/foot, about 1:32. Like Gauge 3 it is almost exclusively for live steam garden railways.
A digression. If you are interested in these larger scales you will come across the term “G Scale” or “LGB”. LGB is actually a brand name for the scale. G Scale is, almost exclusively a Narrow Gauge scale and is Gauge 3 Scale running on Gauge 1 track. So both of these can gain from the existed of G Scale which is probably more popular than either Gauge 3 or Gauge 1. More G Scale in the Narrow Gauge summary.
Gauge O is the original “Gauge 0” (zero), as it was smaller than Gauge 1, but is now universally called “Gauge O” (oh). From being the dominant gauge before the war it gradually lost favour as typical homes, or at least typical homes of those who could afford model railways, got smaller and “OO Scale” arrived. The British version of O Scale is marginally bigger than the USA, largely for the same reasons as outlined in the OO Scale section, below.
Gauge O was often clockwork and later electric, you can still see these early models running at some model railway shows. After the war it became the province of kit and scratch builders, often with hand built track. In more recent years the major hobby manufacturers have started to introduce ready-to-run models, making it more accessible to all. It is common to see both indoor and outdoor layouts in O Gauge, nearly always electrically driven but live steam also exists.
S Scale is a scratch builders scale in the UK, there are a very minimal number of kits and no ready-to-run products. There is however a strong society and a number of specialist suppliers providing track components, wheels and a few other parts. It is the only scale that does not use metric measurements, being 3/16” to the foot, 1:64. Track gauge is 0.884”. For comparison though this equates to approximately 4.75mm/foot and a track gauge of 22.45mm.
Although an uncommon scale in the UK it has very wide support in the USA with many commercial suppliers, including ready-to-run trains.
HO Scale is not strictly the next in terms of size but is easiest explained next. In 1922 the German firm “Bing” developed a range of models 1/2 the scale of O Gauge, hence HO. This has become the most popular scale worldwide except in the UK where it is almost exclusively used for models of overseas railways. There is extremely limited trade support for British outline locomotives or other rolling stock. A very specialist scale for British prototypes.
OO Scale is the dominant scale in the UK. The problem for model makers was that British outline rolling stock, and locomotives in particular, had a smaller “loading gauge”, that is to say they were smaller. Electric motors at the time were simply too big to fit in the reduced size, so they were made slightly bigger, 4mm/foot instead of 3.5mm/foot. However to keep costs down and to be able to re-use existing designs made for HO the track gauge was maintained at 16.5mm, resulting in the anomaly that we still see today, almost 100 years later. This situation left the door open for modellers to create more exacting standards and is why we have EM and P4 Gauge today.
Although primarily an indoor gauge there are many successful garden railways using OO gauge.
EM Gauge was first introduced in 1955 by a group of modellers keen to improve the scale/gauge combination of OO. Initially the gauge chosen was 18mm, eighteen millimetres, hence EM, but this was later increased to 18.2mm. This gives a gauge of 4’ 6.5”, so about 2” narrower than the prototype. The EM Gauge Society also introduced finer wheel standards to match the track. There is specialist trade support for EM Gauge but no ready-to-run products from the major manufacturers, although through a collaboration with Peco, plain track and some points are now available off the shelf.
P4 or Protofour was first formalised in the mid sixties and is intended to be “dead scale”, and exact duplication, of the prototype and therefore comes with a track gauge of 18.83mm. Allied to this are much finer standards for other parts of the track and of the wheels. Today the standard is maintained and supported by the Scalefour Society and, like EM, there is specialist trade support but no ready-to-run products.
TT Scale was introduced by Tri-ang in the 1950’s. Seen as an ideal table top product, hence “TT”. In reality the advent of N SCale and the popularity of OO Scale squeezed TT out of the market. The “Three Millimetre Society” was formed in 1965 and today provides support to people who model in this scale. The Society supplies a small range of kits as well as wheels and other parts with very limited commercial trade support. It is primarily a scratch builders scale.
TT is another scale to suffer the British loading scale issue. TT was originally 1:120, and remains that in most parts of the world, British TT 1s 1:101.6. In addition the original track, and still commonly used, had a gauge of 12mm as this was widely available on the continent as a narrow gauge for HO scale. However, modellers now also used the much more prototypically correct gauge of 14.2mm. Both are supported by the society.
Again a scale with significantly more support outside the UK, especially in Europe.
N Scale was first seen as commercial product by Arnold in Germany in 1962. Called “N Scale” as the track had a gauge of Nine millimetres. Widely supported in the UK, and around the world, it also suffers from the British loading gauge problem, albeit many years after the problem in O and HO arose. Therefore, although most of the world use 1:160 for the scale it is, normally, 1:148 in the UK, resulting in a slightly narrow track gauge of 9mm.
In the USA Lone Star introduced “OOO” Scale push along diecast models the year before Arnold. The “OOO” was a nod toward OO but they were marginally smaller than N with a track gauge closer to 8mm. As electrified models were introduced the gauge was widened to 9mm but N Scale quickly came to dominate this modelling space.
2mm FS is so called as it is a Fine Scale version of N Scale. However, it is a bit more than that. 2mm FS used a precise 2mm/foot scale which is actually 1:152 but the difference from N Scale’s 1:148 is barely noticeable. It uses a track gauge of 14.2mm. As with other similar specialist gauges there is a society providing support and specialist suppliers providing key parts but no ready-to-run stock. Although it has its share of scratch builders many are content to convert N Scale products.
Z Scale was introduced by Marklin of Germany in 1972. With a track gauge of 6mm and a scale of 1:220 or approximately 1.4mm/foot, these are very small models. There are some major suppliers but largely in Germany and the USA, there is no British outline support. There are some people producing resin models to go on appropriate, or nearly appropriate chassis from other countries but it is definitely a challenge to model British outline. Presumably Z Gauge was so called as it was at the end, the smallest you get, however…
T Scale is really small. Introduced in Japan in 2007 it is so called as it runs on Three millimetre track. This approximates to 1:450 or 0.68mm/foot. There is a UK based supplier and there are a couple of UK prototype locomotives but it is very small and a different style of modelling from the conventional. It seems unlikely we will see anything smaller.
You will have noticed that some of these use the term Scale and some Gauge. Unfortunately, and sometimes confusingly, these terms are often used interchangeably but they are not. Primarily this is about Scale, i.e. size, rather than Gauge, i.e. track width but for EM and the original scales the term gauge is always used.