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 Merseyrail in Liverpool, the S Lines in Milan, BART in the San Francisco Bay Area, and many other urban rail networks around the world.