OO9
OO9 is a model railway scale and gauge combination of 4 mm scale and 9 mm (0.354 in) gauge tracks, which models a prototype track gauge of 2 ft 3 in (686 mm). It is a common choice in the United Kingdom for the modelling of narrow gauge railways whose prototype gauges lie approximately between 2 ft (610 mm) and 2 ft 6 in (762 mm). The 9 mm (0.354 in) track gauge is used by N gauge model railways, a common commercial scale, which means that a selection of wheels and mechanisms is readily available.
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