RER C

RER C is one of the five lines in the Réseau Express Régional (English: Regional Express Network), a hybrid commuter rail and rapid transit system serving Paris and its suburbs. The 186-kilometre (116 mi) line crosses the region from north to south. Briefly, between September 1979 and May 1980, the line was known as the Transversal Rive Gauche. The line is operated by SNCF.

The line runs from the northern termini Pontoise (C1), Versailles-Château-Rive-Gauche (C5) and Saint-Quentin-en-Yvelines (C7) to the southern termini Massy-Palaiseau (C2), Dourdan-la-Forêt (C4) and Saint-Martin d'Étampes (C6).

The RER C line is the second-longest in the network, 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 slow and inefficient. The numerous old curves and steep grades on RER C mean trains sometimes need to slow down to 30 km/h (20 mph) to safely pass sections with tight alignments. In contrast, RER A was constructed to 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 can be faster than taking the RER C with its 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 small incidents.