Editing Paris Métro (section)

=== 1960–1990: development of the RER ===
[[File:Viaduc Austerlitz Paris 18.jpg|thumb|The [[Viaduc d'Austerlitz]], crossing the [[Seine]], is used by Line 5]]
The network grew saturated during the 1950s. Outdated technology limited the number of trains, which led the RATP to stop extending lines and concentrate on modernisation. The [[MP 51|MP&nbsp;51]] prototype was built, testing both [[rubber-tyred metro]] and basic automatic driving on the ''[[Voie des Fêtes and the voie navette of the Paris Métro|voie navette]]''. The first replacements of the older Sprague trains began with experimental articulated trains and then with mainstream rubber-tyred Métro [[MP 55|MP&nbsp;55]] and [[MP 59|MP&nbsp;59]], some of the latter still in service (Line 11). Thanks to newer trains and better signalling, trains ran more frequently.

The population boomed from 1950 to 1980. Car ownership became more common and suburbs grew further from the centre of Paris. The main railway stations, termini of the suburban rail lines, were overcrowded during rush hour.{{cn|date=February 2024}} The short distance between Métro stations slowed the network and made it unprofitable to build extensions. The solution in the 1960s was to revive a project abandoned at the end of the 19th century{{cn|date=February 2024}}: joining suburban lines to new underground portions in the city centre as the [[Réseau Express Régional]] (regional express network; RER).

[[File:Saint-Lazare-9-Salle-echang.jpg|thumb|[[Saint-Lazare (Paris Métro)|Saint-Lazare station]]]]
The RER plan initially included one east–west line and two north–south lines. RATP bought two unprofitable SNCF lines—the Ligne de Saint-Germain (westbound) and the Ligne de Vincennes (eastbound) with the intention of joining them and to serve multiple districts of central Paris with new underground stations. The new line created by this merger became Line&nbsp;A. The Ligne de Sceaux, which served the southern suburbs and was bought by the CMP in the 1930s, would be extended north to merge with a line of the [[SNCF]] and reach the new [[Charles de Gaulle Airport]] in Roissy. This became [[RER B|Line&nbsp;B]]. These new lines were inaugurated in 1977 and their wild success outperformed all the most optimistic forecasts to the extent that line A is the most used urban rail line in Europe with nearly 300&nbsp;million journeys a year.{{cn|date=January 2024}}

Because of the enormous cost of these two lines, the third planned line was abandoned and the authorities decided that later developments of the RER network would be more cheaply developed by the SNCF, alongside its continued management of other suburban lines. However, the RER developed by the SNCF would never match the success of the RATP's two RER lines. In 1979, the SNCF developed [[RER C|Line C]] by joining the suburban lines of the [[Gare d'Austerlitz]] and [[Gare d'Orsay]], the latter being converted into a museum dedicated to impressionist paintings. During the 1980s, it developed [[RER D|Line&nbsp;D]], which was the second line planned by the initial RER schedule, but serving Châtelet instead of [[République (Paris Métro)|République]] to reduce costs. A huge Métro-RER hub was created at [[Châtelet–Les Halles]], becoming one of the world's largest underground stations.<ref>{{cite book|last1=Aplin|first1=Richard|last2=Montchamp|first2=Joseph|title=Dictionary of Contemporary France|url=https://books.google.com/books?id=DGS2AgAAQBAJ&pg=PT109|year=2014|publisher=Routledge|isbn=978-1-135-93646-4|page=109}}</ref>

The same project of the 1960s also decided to merge Line 13 and Line 14 to create a quick connection between Saint-Lazare and Montparnasse as a new north–south line. Distances between stations on the lengthened line 13 differ from that on other lines in order to make it more "express" and hence to extend it farther in the suburbs. The new [[Paris Métro Line 13|Line&nbsp;13]] was inaugurated on 9 November 1976.