RER B is one of the five lines in the Réseau Express Régional, a hybrid commuter rail and rapid transit system serving Paris, France and its Île-de-France suburbs. The 80-kilometre (50 mi) RER B line crosses the region from north to south, with all trains serving a group of stations in central Paris, before branching out towards the ends of the line.

The Réseau Express Régional, commonly abbreviated RER, is a hybrid commuter rail and rapid transit system serving Paris and its suburbs. It acts as a combined city-center underground rail system and suburbs-to-city-center commuter rail. In the city center, it acts as a faster counterpart of the Paris Métro, having fewer stops.