The Réseau Breton (RB) is a 1,435 mm, standard gauge, and former 1,000 mm, metre gauge, railway in Finistère, France, with a few kilometres of line in Côtes d'Armor, Ille-et-Vilaine and Morbihan. The hub of the system was Carhaix. The metre gauge lines were built with the capacity to be easily converted to standard gauge if necessary.