Paris Métro Line 4

Line 4 (French pronunciation: [liɲᵊ katʁᵊ]) is one of the sixteen lines of the Paris Métro rapid transit system and one of its three fully automated lines. Situated mostly within the boundaries of the City of Paris, it connects Porte de Clignancourt in the north and Bagneux-Lucie Aubrac in the south, travelling across the heart of the city. Until its southern terminus was changed from Porte d'Orléans to Mairie de Montrouge in 2013, the line was sometimes referred to as the Clignancourt – Orléans Line. At 13.9 km (8.6 mi) in length, it connects with all Paris Métro lines apart from the very short 3bis and 7bis branch lines, as well as with all 5 RER express lines. It also serves three of the Paris Railway stations, Gare du Nord, Gare de l'Est, and Gare Montparnasse. It is the second-busiest Métro line after Line 1, carrying over 154 million passengers in 2004.

Line 4 was the first line to connect to the south side of the River Seine, through an underwater tunnel built between 1905 and 1907. Line 4 long ran the oldest cars still in service on the system, the MP 59. They used rheostats to dissipate the braking power through resistance and made it the hottest line in the system. Those trains were withdrawn from service during the course of 2011 and 2012 after 45 years (with some being in service for 50 years). They were replaced by the MP 89 CC stock transferred from Line 1, once new driverless trains made those redundant on that line. (From fr:Ligne 4 du métro de Paris).

In 2013, Line 4 was extended for the first time since its initial construction, into the southern suburbs of Montrouge. The line was further extended to Bagneux–Lucie Aubrac in 2022, connecting to the future Grand Paris Express. The line has been retrofitted for full automation, with the first automated trains commencing service on 12 September 2022.