The RER (; French: Réseau Express Régional, "Regional Express Network") is a rapid transit system in France serving Paris and its suburbs. The RER is an integration of a modern city-centre underground rail and a pre-existing set of commuter rail lines. It has several connections with the Paris Métro within the city of Paris. Within the city, the RER functions like the Métro, but as it has fewer stops, the system acts as one of express trains. Since 1999 the network has consisted of five lines: A, B, C, D and E. The RER is still expanding; Line E, which opened in 1999, is planned for westward extension by 2020.