In rail transport modelling, Sn3ý is a scale/gauge combination derived from S scale to represent narrow gauge track by using gauge track (the same as HO gauge). The scale is .
Sn3ý is popular in South Africa, Australia (particularly Western Australia, Queensland and Tasmania where narrow gauge systems exist) and New Zealand.
Sn3ý is very rarely or never used for modelling in other countries with 3 foot 6 (1067mm) gauge railways such as in Japan, Taiwan, and Indonesia
As track scales down to 16.5 mm at 1:64, modelers use HO gauge track (which represents standard gauge at 1:87 scale) on Sn3ý layouts.
Sn3ý is the primary scale for modeling New Zealand's narrow gauge 3 ft 6 in railways. The majority of rolling stock available, are white-metal kits, making them considerably more expensive and heavier than other scales and countries. Many of these kits are highly detailed. Buildings are generally hand-made, and track (HO gauge track) can either be purchased or hand laid with sleepers and rail. Alternatives for modeling New Zealand railways is 1:120 or TT scale, known as NZ120, as it is a cheaper option. HOn3ý gauge, HO scale with 12mm gauge, is also increasing in popularity.
Sm gauge is also defined by the German NEM 10 standard, representing Continental European metre gauge.