Réseau Express Régional

The Réseau Express Régional (French pronunciation: [ʁezo ɛkspʁɛs ʁeʒjɔnal]; English: Regional Express Network), commonly abbreviated RER (pronounced [ɛʁəɛʁ]), is a hybrid commuter rail and rapid transit system serving Paris and its suburbs. It acts as a combined city-center underground rail system and suburbs-to-city-center commuter rail, similar to the Elizabeth line in London, S-Bahn in German-speaking countries and the S Lines in Milan.

Conceived of as a métropolitain express ('express metro') during the mid-1930s, the scheme was revived in the 1950s and construction began in the early 1960s. The RER was not fully conceptualised until the completion of the Schéma directeur d'aménagement et d'urbanisme (roughly: 'master plan for urban development') in 1965. The RER network, which initially comprised two lines, was formally inaugurated on 8 December 1977 in a ceremony that was attended by President Valéry Giscard d'Estaing. A second phase of construction commenced at the end of the 1970s which saw additional lines constructed along with extensions to the original two. The RER is operated partly by RATP, the authority that operates most of the public transport in Paris, and partly by the SNCF, France's national rail operator.

As of 2023, 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. The performance of the RER has made it a model for proposals to improve transit within other cities. In November 2022, French President Emmanuel Macron announced the creation of additional RERs that will serve the ten largest French metropolises by 2040.