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 the city centre of Paris and its suburbs. It functions as a combined city centre underground rail system and suburbs-to-city-centre commuter rail network, in a similar manner to the S-Bahn systems in Germany, Austria and Switzerland, S-train in Copenhagen, the Pendeltåg in Sweden, the Overground, Thameslink and Elizabeth Line in London, the S Lines in Milan, and many other commuter rail networks around the world. It is part of the network of transport services managed by Île-de-France Mobilités.

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 (rough translation: '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 Group, the authority that operates most of the public transport in Paris, and partly by the SNCF Voyageurs, France's national rail operator.

As of 2025, the network consists of five lines: A, B, C, D and E. The network has 257 stations and has interchanges with the Métro, tramway 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 continues to expand: RER E, which opened in 1999, is planned for westward extension toward La Défense and Mantes-la-Jolie in two phases between 2024 and 2026. The performance of the RER has made it a model for proposals to improve public transport within other cities. In November 2022, the French President Emmanuel Macron announced the creation of additional RER systems that will serve the ten largest French metropolises by 2040.