RER A

RER A is one of the five lines in the Réseau Express Régional (English: Regional Express Network), a hybrid commuter rail and rapid transit system serving the city and suburbs of Paris, France. The 108.5-kilometre (67.4 mi) line crosses the region from east to west, with all trains serving a group of stations in central Paris, before branching out towards the ends of the line.

The initial portion of the line was built in stages between December 1969 and December 1977 by connecting two existing suburban commuter rail lines with a new tunnel under Paris: the line between Vincennes and Boissy-Saint-Léger in the east (which formerly terminated at the now-closed Gare de la Bastille), and the line between Saint-Germain-en-Laye and Nanterre in the west (which formerly used a surface alignment to the Gare Saint-Lazare that is still in use as Transilien L). The viaduct between Vincennes and the former Gare de la Bastille terminus was redeveloped into the Promenade plantée elevated park in 1993.

Since its opening, three additional branches have been added: one in the east serving Marne-la-Vallée and Disneyland Paris, and two to the west serving Poissy and Cergy.

The RER A has had a significant social impact on Paris and the surrounding region by speeding up trips across central Paris (by making far fewer stops than the Paris Métro) and by bringing far-flung suburbs within easy reach of the city centre. The line has far exceeded all traffic expectations, currently serving over 1.2 million passengers per day, on about 300 million journeys per year. It is one of the busiest rapid transit lines in Europe.