The Ferrocarril de Sóller, acronym FS, is an interurban railway and the name for the company which operates the electrified 3 ft narrow gauge tracks running between the towns of Sóller and Palma on the Spanish island of Mallorca.

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).