3 mm scale

3 mm scale, also known as 3 mm finescale, is a model railway scale of 3 mm : 1 ft (305 mm) used for British prototypes. Introduced as British TT gauge, it sits approximately halfway between British N gauge and OO gauge but is not as popular as either and there is no longer any mass manufacturer ready-to-run support. When TT gauge model railways were developed for British prototypes, in order to fit the small British prototypes, the scale was enlarged but without altering the 12mm gauge. The result, British TT gauge, is too narrow. This led to the development of 14.2 mm (0.559 in) gauge 3mm finescale. Thus two finescale standards were developed. By far the more common of these is 14.2 mm gauge track, which is accurate. Some modellers choose to use slightly narrower 13.5 mm track due to the necessary oversize motion of outside-cylindered steam locomotives.

British TT, also known as TT3, was pioneered by Triang Railways as ready-to-run models.

Other gauges are also used to model other prototypes (Irish broad gauge, Brunel gauge). For narrow gauges, N gauge 9 mm track represents 3 ft (914 mm) gauge; Z gauge 6.5 mm (0.256 in) track represents to 2 ft 2 in (660 mm) gauge.

The 3mm Society caters for all modellers of 3 mm scale.