The RER (Réseau Express Régional, IPA /ɛʀøɛʀ/, "Regional Express Network") is an urban rail network in the Île-de-France région, notably Paris and its agglomeration.
The RER is a hybrid system, being an integration of modern city-centre subway and pre-existing regional rail. Within the city of Paris, the RER serves as an express network offering multiple connections to the Paris Métro. The essential central part of the RER was completed by a massive civil engineering effort between 1962 and 1977, and features some unusually spacious deep stations. As of 2007 the network comprises five lines: A, B, C, D and E. The RER is still expanding today — Line E was inaugurated in 1999, and seems a likely candidate for future extension.
The RER currently serves 256 stops and runs over 587 km (365 miles) of track, including 76.5 km (48 miles) underground. 33 stops are served inside the city of Paris itself. Each line crosses the city proper almost exclusively underground and on dedicated tracks. Due to its hybrid nature, some parts of RER lines are operated by the city transport authority managing the Métro (RATP) and others by the national rail company (SNCF). In 2004, 782.9 million journeys were made on the five lines of the RER.citation needed]