High Quality Content by WIKIPEDIA articles The RER (French pronunciation: French: R seau Express R gional, "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 M tro within the city of Paris. Within the city, the RER functions like the M tro, but as it...