Edinburgh & Lothians
Miniature Railway Club

A common question we are asked at exhibitions, particularly with a narrow gauge layout, is about its scale and gauge. We hope the following explanations will help.

Scale:

This is ratio of the size of the model compared to the prototype. e.g. In 'OO' the Scale is 1:76 meaning the prototype is 76 times large than the model (1).
In Britain, at least, the scale is also often given in the form (again for 'OO') 4mm:1 foot: i.e. 4mm on the model represents one foot on the prototype (2).

Gauge:

In railway terms there are a number of gauges, but the one normally referred to is the rail gauge (distance between the rails). Standard gauge is 1435mm (4 feet 8 1/2 inches). Less than this is narrow gauge, wider is broad gauge. This means that technically in many countries the main railway gauge is either narrow gauge e.g. South Africa - 1067mm (3 feet 6 inches) or Broad Gauge e.g. Ireland - 1600mm (5 feet 3 inches).

Note: The terms OO, HO etc. are not really scales or gauges, but a name for a scale/gauge combination.

Narrow Gauge Models:

For narrow gauge models there are internationally at least three systems used for describing them. These can be summed up as follows, using 0 scale models on 16.5mm gauge track as an example:

British System:
0:16.5 - Uses the scale name in combination with the model gauge in millimetres.

American System (Also appears to be used in Australia, New Zealand etc.):
0n30 - Uses the scale name in combination with the prototype gauge in inches, the n standing for narrow. In some cases the gauge is referred to in feet, e.g. On3 for a 3 feet gauge line.

European System:
0e - Uses the scale name with a code letter, in this case e for eng (German for narrow). In the European system the letters used appear to be:

Britain's 'odd' scales:

Britain has a habit of using a scale slightly larger than is accurate for the corresponding track gauge (see the table below). The generally accepted reason is to do with the smaller size of British rolling stock. As each smaller scale was introduced (usually from Europe) it was necessary to enlarge the size of locomotives to allow the available motors to fit. The larger scale also allowed more clearance between wheels and valve gear and gave room for bogies to swing on curves, which are usually much tighter than scale. Today with smaller motors and finer wheel standards some of these reasons are less relevant, but the scales are too well established to change.