It is important to understand the difference between gauge and scale.

A model trains "Scale" is the relationship between its size and the size of an actual train.

A model trains "Gauge" is the distance between the rails.

The more popular scales and gauges are discussed below.

2mm Scales.

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

2mm Scale
2mm to 1ft, 1:152.3 scale
9.42mm 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.

TT Gauge
3mm to 1ft, 1:101.6 scale
12mm 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 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 be very obvious.

00 Gauge
4mm to 1ft, 1:76 scale
16.5mm gauge (inside track width)
This is the most popular scale for British modellers and is probably the best supported in the industry. However the track gauge is too narrow to be accurate.

EM Gauge
4mm to 1ft, 1:76 scale
18.2mm 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 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 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 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, 1:43.5 scale
32mm 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
This is the scale track option for 7mm scale.

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