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The RER (pronounced "air-ee-air" in French) is the high-speed urban "super-metro" network in and around Paris in France. The initialism stands for Réseau Express Régional: Regional Express Network.

It currently consists of 5 lines, A to E, and interconnects with the Paris Metro. It is run jointly by the RATP (the body that runs most public transport in Paris including the buses and the metro) and SNCF (French national railways).

Its use of standard gauge and overhead power supply means that trains can run on existing urban railway lines. Most of the lines are the result of connecting existing rail lines that branch out from the rail termini through new underground tunnels across Paris. Some lines use double decker trains to achieve greater capacity.

After many years of planning, construction began on the network on July 6, 1961 with line A. The first segments to be built new involved the extension of the old Saint-Germain-en-Laye line, re-electrified using an overhead system (it had originally been electrified using third rail) on a new alignment from Nanterre through La Défense and the new stations of Auber (at the time of its construction the largest underground station in the world) and Châtelet-Les-Halles in the heart of Paris. The extension to Gare de Lyon joined up the existing Vincennes line (Bastille to Boissy-Saint-Léger), completing the original line A. A branch of line A from Vincennes to Marne-la-Vallée was built new. This was subsequently extended to Disneyland Resort Paris.

Line B resulted from connecting the Luxembourg terminus of the Ligne de Sceaux with the Gare du Nord main line railway terminus through Châtelet-Les-Halles.

Line C was created by connecting the Gare d'Orsay railway terminus (the present-day Musée d'Orsay) with the Invalides terminus of the Rive Gauche line to Versailles along the banks of the Seine.

Line D connected the Gare du Nord with the Gare de Lyon via Châtelet-Les-Halles.

Finally, the construction of line E in 1998 between the termini of Gare Saint-Lazare and Gare de l'Est left the Gare Montparnasse the only main line terminus in Paris not directly connected to the RER system, the TGV line out of Montparnasse being served by the RER through the suburban hub of Massy Palaiseau (see Ligne de Sceaux).

See also Transportation in France.