Sometimes the terms “Scale” and “Gauge” are used interchangeably even though, technically, they are different.

The Scale of a model train, refers to its size in relation to its real-world prototype. A model is reduced by a specific proportion. For example, a model locomotive that is 1/48th the size of the real thing is called 1/48th or 1:48 scale. (As it happens, O gauge trains are 1/48th scale.)

The Gauge is simply the distance between the running rails.

Standard gauge track is 1435.5mm wide, and when reduced to a scale of 1:87, the model track becomes 16.5mm. This is called HO gauge, the most common gauge used by modellers. Several attempts were made during the twentieth century to develop a universal compact system for model trains. This has only added to the diversity of Gauges that now exist. This can be interpreted as a good thing as everyone requires different size models and gauges depending on their individual circumstances.

The various gauges are detailed below.

1 Gauge
Track width: 45mm
Scale: 1:32 (10mm to 1ft/305mm)
This is the earliest of the miniature railway sizes. It is also the largest after LGB narrow gauge models, which run on gauge 1 track. Any models larger than gauge 1 are generally considered to come under the heading of model engineering rather than model railways. The Germany manufacturer Marklin was a major maker, using the lithographed tin plate techniques of the time.

O Gauge
Track width: 32mm
Scale: 1:43 (Great Britain and France), 1:45 (rest of Europe), 1:48 (USA); 7mm, 6.6mm, and 6.35mm to 1ft/305mm respectively.
O gauge was introduced in the early 1900’s in an attempt to produce a more compact system. Even though the linear scale was interpreted slightly differently in various countries, in practice the discrepancies are very small. The famous Hornby tin plate of yesteryear are O gauge models and are still keenly sought after.

S Gauge
Track width: .844in
Scale: 1:64 (4.8mm to 1ft/305mm)
S gauge was originally called H (half gauge 1) when, earlier in the twentieth century, it was introduced as another attempt to develop a more compact scale. In Great Britain, a small band of enthusiasts keeps the gauge active, whilst in the United States of America there is substantially more trade backing.

HO Gauge
Track width: 16.5mm
Scale: 1:87 (3.5mm to 1ft/305mm)
This gauge is by far the most important commercially. It was evolved in the 1930’s in yet another attempt to establish a compact miniature system. It is, as its name suggests, half of O scale and gauge. It has been estimated that about 80% of the model railway industry is dedicated to producing HO models.

OO Gauge
Track width: 16.5mm
Scale: 1:78 (4mm to 1ft/305mm)
In Great Britain, early kit and model makers found it difficult to fit British-outline superstructures over the motors and chassis used by the early German manufacturers for their HO models. To overcome this, the British designers enlarged the scale to 4mm to 1ft (305mm) – but they retained the HO gauge track of 16.5mm.
This discrepancy is substantial, as it is equivalent to running on prototype track about 8 1/2in (216mm) narrower than standard gauge. Despite this, OO has become the most common in Britain, with manufacturers supplying models of all kinds.

TT Gauge
Track width: 12mm
Scale: 1:120 (2.5mm to 1ft/305mm)
TT stands for ‘Table Top’. It was introduced in the USA in the 1950’s, and for a time ready-to-run models were produced commercially in countries such as Britain (where a scale of 1:100 or 3mm to 1ft/305mm was used).

N Gauge
Track width: 9mm
Scale: 1:160 (1.9mm to 1ft/305mm)
In Great Britain 1:148 (2.05mm to 1ft/305mm)
After HO and OO, this is the most popular size. A large selection of ready to run models is available, covering all the major countries for fractionally larger in scale terms, again to accommodate foreign motors and chassis in British bodies. The small size on N gauge means that good layouts can be achieved in a fairly small area, making it a very attractive proposition for anyone with limited space.

Z Gauge
Track width: 6.5mm
Scale: 1:220 (1.6mm to 1ft/305mm)
Z is the smallest commercially produced gauge. The minute models have a unique appeal. In this tiny size, a good layout could be built on a tea tray or in a desk drawer!

As detailed above, gauge refers to track size.

All true models are reduced replicas of full-sized originals (which railway modellers call the prototype). in relation to its prototype – the exact deduction is expressed mathematically as 1:32 (for example). This is the scale of the model.

The Gauge is simply the distance between the running rails.