Cergy is a commune in the French department of Val-d'Oise, to the northwest of Paris. It is located 27.8 km (17.3 mi) from the centre of Paris, in the "new town" of Cergy-Pontoise, created in the 1960s, of which it is the central and most populated commune.

RER A is one of the five lines in the Réseau Express Régional, a hybrid commuter rail and rapid transit system serving Paris, France and its suburbs. 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.