|Termini||Pontoise (C1), Versailles-Château-Rive-Gauche (C5), Saint-Quentin-en-Yvelines (C7)|
Massy-Palaiseau (C2), Dourdan-la-Forêt (C4), Saint-Martin-d'Étampes (C6), Versailles-Chantiers (C8)
|Type||Rapid transit/commuter rail|
|System||Réseau Express Régional|
|Rolling stock||Z 5600, Z 8800, Z 20500, Z 20900|
|Ridership||140,000,000 journeys per year|
|Opened||26 September 1979|
(last extension in 2006)
|Line length||185.6 km (115.3 mi)|
The line runs from the northwestern termini Pontoise (C1), Versailles-Château-Rive-Gauche (C5) and Saint-Quentin-en-Yvelines (C7) to the southeastern termini Massy-Palaiseau (C2), Dourdan-la-Forêt (C4), Saint-Martin d'Étampes (C6) and Versailles-Chantiers (C8).
The RER C line is the second-longest in the network, with over 187 km (116 mi) of route. RER C was created from an amalgamation and renovation of several old SNCF commuter lines unlike RER A and B which had newer sections owned and constructed by RATP. Each day, over 531 trains run on the RER C alone, and carries over 540,000 passengers daily, 150,000 passengers more than the entirety of the TGV network.
It is the most popular RER line for tourists, who represent 15% of its passengers, as the line serves many monuments and museums, including the Palace of Versailles. However, the numerous stops, combined with the old and fragile infrastructure the line inherited, makes the Parisian section of the RER C very slow and inefficient. The numerous old curves and steep grades on RER C means trains sometimes need to slow down to 40 km/h (25 mph) to safely pass sections with tight alignments. In contrast, RER A was constructed with more modern standards enabling much higher average operating speeds. These problems are particularly evident on trips to and from the northern suburbs to the city center as taking Transilien lines and transferring to the Métro is much faster than taking the meandering RER C with closely spaced stops. In addition, the RER C's complicated operating schedule created by its complex network of numerous branches means the entire line is vulnerable to delays from even the smallest incidents.
These issues have led to the line being called "Réseau Escargot Régional" (Regional Snail Network) by the local populace.
Line C was opened on 26 September 1979, following the construction of a new 1-kilometre (0.62 mi) tunnel connecting the Gare d'Orsay railway terminus (now Musée d'Orsay) with the Invalides, terminus of the Rive Gauche line to Versailles, along the banks of the Seine. Services operated between Versailles-Château-Rive-Gauche - Invalides - Quai-d'Orsay, branching to Massy - Palaiseau, and Juvisy - Dourdan / Saint-Martin d'Étampes. At that point the line was named the Transversal Rive Gauche.
In May 1980, service was extended to Saint-Quentin-en-Yvelines - Versailles - Chantiers - Gare des Invalides. The RER C designation was then only introduced from this point onwards, replacing the Transversal Rive Gauche name.
On 25 September 1988, the VMI ("Vallée de Montmorency - Invalides") branch to the north-west opened. This branch mostly used the infrastructure of the "ligne d'Auteuil" (incorporated into the "ligne de petite ceinture" from 1867, closed to passengers from 22 July 1934), and a new 3-kilometre (1.9 mi) tunnel connection between Batignolles and St-Ouen, connecting to the RER C's main trunk at Champ de Mars-Tour Eiffel via a curved bridge (the only one in Paris) over the Seine river. That extended services to Montigny - Beauchamp and Argenteuil.
A further 9 kilometres (5.6 mi) extension from Montigny - Beauchamp to Pontoise was opened on 28 August 2000. On the same day a new station, Bibliothèque François Mitterrand, opened in order to create a new connexion with Métro Line 14, located between Paris-Austerlitz and Boulevard Masséna, which was closed and replaced by the new station.
In February 2012, Versailles - Rive Gauche was renamed Versailles-Château-Rive-Gauche, to avoid frequent tourist confusion with other stations in Versailles.