The Réseau Express Régional (English: Regional Express Network), commonly abbreviated RER (French pronunciation: [ɛʁ.ə.ɛʁ]), is a hybrid commuter rail and rapid transit system serving Paris and its suburbs. It acts as a combined city-centre underground rail system and suburbs-to-city-centre commuter rail. In the city centre it acts as a faster counterpart of the Paris Métro, having fewer stops. This has made it a model for proposals to improve transit within other cities. The network consists of five lines: A, B, C, D and E. The network has 257 stations and has interchanges with the Métro and commuter rail within the City of Paris and the suburbs. The lines are identified by letters to avoid confusion with the Métro lines, which are identified by numbers. The network is still expanding: RER E, which opened in 1999, is planned for westward extension toward La Défense and Mantes-la-Jolie in two phases by 2024–2026. From Wikipedia
