Wikipedia clarifies: "The Reseau Express Regional, colloquially called the RER, is a hybrid commuter rapid transit system in France serving Paris and its suburbs. The RER combines a modern suburb-city centre underground rail system and commuter rail lines. It has several connections with the Paris Metro within the city of Paris. Also inside the city, the RER functions much like the Metro, but is faster as it has fewer stops. The network consists of five lines: A, B, C, D and E. The lines are identified by letters to avoid confusion with the Metro lines, which are identified by numbers. The RER is still expanding: Line E, which opened in 1999, is planned for westward extension in two phases by 2020-2022."
This photo was sent in by KW. He notes that "you can study the map of the RER line while using one of these urinals."