The association covers modelling British narrow-gauge railways in the following gauges:
However other gauges and O scales are also accepted. For instance members modelling American narrow gauge often use On30, and Oe when modelling European prototypes. While many members model British railways, prototypes from all round the world are catered for, including the railways of countries that were part of Britain's former empire.
The association was founded in 1979.
The current membership of the association is in excess of 1000 members. Most are resident in the United Kingdom, but about 10% live overseas. The association has 6 local groups in different areas of Britain, and two groups based outside Britain.
The association publishes a bi-monthly magazine, Narrow Lines, which is posted free to all members. Additionally the association has published a series of 5 handbooks on various aspects of modelling, and a series of 12 booklets containing scale drawings of various items of narrow-gauge rolling stock.