The Paris RER is the rapid transit system that is useful when you are trying to find things to do in Paris located outside of the city center. If you are traveling within zones 1-3 then access is free if you have a Paris Métro pass. For traveling further outside of the city
The Paris RER contains 257 stations, 33 of which are within the city of Paris, and runs over 587 km (365 mi) of track, including 76.5 km (47.5 mi) underground. Each line passes through the city almost exclusively underground and on dedicated tracks. The RER is operated partly by RATP, the transport authority that operates Paris Métro, and partly by SNCF, the national rail operator. Despite this, the system uses a single fare structure and no transfer is needed between parts run by the two operators. Total traffic on the central sections of lines A and B, operated by RATP, is over 450 million.
The Paris RER is a modern city-center underground rail integrated with a pre-existing set of commuter rails. It has several connections with the Paris Métro within the city of Paris. Within the city, the RER functions like the Métro, but is faster as it has fewer stops. 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.