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 (19 mph) to safely pass sections with tight alignments.

[3] In contrast, RER A was constructed with more modern standards enabling much higher average operating speeds.

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.

Services operated between Versailles-Château-Rive-Gauche – Invalides – Quai-d'Orsay, branching to Massy – Palaiseau, and Juvisy – Dourdan / Saint-Martin d'Étampes.

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 Saint-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.

The station Porte de Clichy, located between Pereire – Levallois and Saint-Ouen, opened on 29 September 1991.

[8] However, this vital project to serve the Saint-Quentin-en-Yvelines conurbation (improved service to La Verrière and Trappes stations, and control of (high) ridership at Saint-Quentin-en-Yvelines - Montigny-le-Bretonneux station) has been delayed[9] due to the high infrastructure costs involved, estimated at over €120 million at June 2008 economic conditions.