Model Railway Gauge and Scale Guide
It is important to understand the difference between gauge and scale. Like real-world railways, the gauge is the distance between the rails, with trains and rolling stock built for each particular gauge; scale is the proportion that the size of the model is compared to its real-world equivalent. The scale is normally expressed as a ratio (1:16 or 1/16) or a size (1 inch :1 foot). Sounds simple enough? Not quite! When a model train is scaled down, the gauge is not necessarily to scale, but to the nearest standard gauge. This means that you could have two different trains, both with the same gauge, but a slightly different scale. In practice, this will be hardly noticeable, but it is worth bearing mind.
In decreasing order of size, the most common model railway gauges are:
7mm to 1ft, 1:43.5 scale
4mm to 1 ft, 1:76 scale
OO is by far the most popular gauge, although the gauge is 15% under scale.
3.5mm to 1ft, 1:87 scale
The same gauge as OO, but with the smaller scale the gauge is in correct proportion to the scale.
2mm to 1ft, 1:148 scale
Twice as small as OO gauge