Three foot gauge railways have a track gauge of 3 ft or 1 yard. This gauge is a narrow gauge and is generally found throughout North, Central, and South America. In Ireland, many secondary and industrial lines were built to 3 ft gauge, and it is the dominant gauge on the Isle of Man, where it is known as the Manx Standard Gauge. Modern 3 ft gauge railways are most commonly found in isolated mountainous areas, on small islands, or in large-scale amusement parks and theme parks. This gauge is also popular in model railroading, and model prototypes of these railways have been made by several model train brands around the world, such as Accucraft Trains (US), Aristo-Craft Trains (US), Bachmann Industries, Delton Locomotive Works (US), LGB (Germany), and PIKO (Germany).

In rail transport, track gauge is the distance between the two rails of a railway track. All vehicles on a rail network must have wheelsets that are compatible with the track gauge. Since many different track gauges exist worldwide, gauge differences often present a barrier to wider operation on railway networks.