Paris Metro Line 12 (French: Ligne 12 du métro de Paris : initially opened as Ligne A du Nord-Sud (North-South Line A)) is one of the sixteen lines currently open on the Paris Métro. It links Issy-les-Moulineaux, a suburban town southwest of Paris, to Aubervilliers, in the northeast. With around 54 million passengers per year, Line 12 was the twelfth busiest line of the network in 2021.. It services several major stops, such as Porte de Versailles, Concorde, Madeleine, and two national railway stations, Gare Montparnasse and Gare Saint-Lazare. The service runs every day of the week, and the line uses MF 67 series trains, the network's standard since the early 1970s.
Line 12 was founded in 1905 as Line A by the Nord-Sud Company, who also built Line 13. It was built between 1905 and 1910, to connect the districts of Montparnasse, in the south, and Montmartre, in the north of the French capital. The first trip, from Porte de Versailles to Notre-Dame-de-Lorette, was on 5 November 1910. The line was the second to be built on the north–south axis of the city, in competition with Line 4 of the Compagnie du chemin de fer métropolitain de Paris (CMP; Paris Metropolitan Railway Company). It was extended bit by bit until 1934 when it reached Mairie d'Issy in the south. Tunnelling to the northern terminus at the Porte de la Chapelle on the perimeter of Paris had been completed in 1916. In 1930, the CMP bought the failing Nord-Sud company and Line A was integrated into the new, unified network as Line 12. In 1949, the CMP was itself merged into the RATP, Paris's public transport company, which still operates the line today.
The line was built using cut-and-cover excavation techniques. Since this method cannot be used under buildings, the route follows the streets above. It remains unchanged today and many original design features, such as the Nord-Sud company's refined ceramic decor, remains in the stations. Some stations are decorated thematically: Assemblée Nationale has murals explaining the intricacies of the lower house of the French Parliament, while the tiling at Concorde represents an extract from the Declaration of the Rights of Man and of the Citizen (1789).
History
Planning (1901–1905)
Engineer Jean-Baptiste Berlier proposed to the City of Paris to finance and build a new line linking the areas of Montmartre and Gare Saint-Lazare in the north with Montparnasse and Vaugirard in the south. The line would establish a second north–south transport axis of the city, west of the existing line four. Berlier's design of two deep, parallel, iron-lined tunnels was modelled on the London Underground system. This method allowed the straightest line possible to be built, without passing underneath buildings and free from interference with underground sewers. The City Council liked the plan and on 28 December 1901 granted the tender for a Montparnasse-Montmartre line to Misters Janicot and Berlier. In June 1902, a new company, rival of the "Parisian Metropolitan Rail Company" (CMP), was established : the Nord-Sud Company (in French ). The grant to build what would become Line A was transferred to the Nord-Sud Company; which was approved on 26 March 1907. While the infrastructure of the CMP was financed by the city corporation, Nord-Sud project was the sole responsibility of the Nord-Sud Company.
The link between two important but distant urban centres guaranteed heavy traffic for the line and it would also directly compete with the CMP and tramway companies, threatening their monopoly across the city. The City was wary of inciting new demands to license other lines, and of eventually provoking industrial disorder, something already experienced on the tramway network. French law required a Déclaration d'utilité publique ("Declaration of Public Utility"), a statement of the project's public benefit before municipal construction, the concerns meant that it was not promulgated for the line until 3 April 1905. The law announced the creation "of public utility, of local interest, this establishment, in Paris, of a railroad, electrically powered, dedicated to the transport of passengers and their hand-luggage, from Montmartre to Montparnasse". The law became active on 19 July 1905, the southern terminus was at Porte de Versailles, long, with a northern branch from Gare Saint-Lazare to Porte de Saint-Ouen. On 10 April 1908, the northern extension from Place des Abbesses to Place Jules Joffrin, long, was in turn authorised, followed by the final section to Porte de la Chapelle () on 24 January 1912.
Construction and opening (1907–1910)
The boggy undersoil of Paris made it impossible for the engineers to follow their initial concept of deeply excavated, metal-lined tunnels, foiling Berlier's original idea. Digging deeper to more stable grounds would raise costs, which were at Berlier's own expense. The line was finally established immediately underneath the streets using the cut and cover method of the CMP's lines, the then-standard in use on the Metropolitan system. As a consequence, the layout contains several difficult curves especially in the northern part where narrow streets have a winding route. The line had 23 stations, all with vaulted roof construction. An interchange with lines A and B was established at Saint-Lazare, but there was no connection with the CMP's lines.
Work on the sub-fluvial tunnel underneath the Seine, between the Chambre des Députés station (now known as Assemblée nationale) and Concorde, took place between July 1907 and July 1909. This long section runs through a bed of limestone after a break in the sand on the Rive Gauche. The under-river passage was bored by two early tunnel boring machines, each with an external diameter of . The head of the machine cut into the rock, while in two intermediate chambers 24 hydraulic jacks exerted a pressure of on the rock walls so the machine could advance. At the back, a mobile arm turned on the axis of the tunnelling shield to install panels on walls of the tunnel.
The two tubes, with an internal diameter of , were made from thick rings of cast iron. Each ring was constructed from ten panels, with one counter-key panel and a key panel to lock the segments in place. The casing extended downstream and upstream. The two tubes are not parallel: the distance between them varies from . Work was delayed by the 1910 Great Flood of Paris. During the works, the future Chambre des Députés station housed an air and water compressor. The compressed air prevented the collapse of the digging shield from the weight of the waters above. The tubes, initially unlined apart from the cast iron rings, were covered with an internal protective layer of masonry in 1920.
The section from Porte de Versailles to Notre-Dame-de-Lorette was inaugurated on 4 November 1910 and opened to the public on the next day. Porte de Versailles was set up as a terminus station with a connection to a workshop. The latter was joined by to the Petite Ceinture rail line, which allowed for the transfer of trains between lines. At the other end, the provisional terminus at Notre-Dame-de-Lorette was set up with three tracks including a central depot.
On the opening day, public representatives rode from Notre-Dame-de-Lorette to Porte de Versailles, and returned to Gare Saint-Lazare for a buffet held in the rotunda. The press reports were laudatory, citing the journey's smoothness, and the stations' brightness. From the beginning, the traffic on the line was significant and the quantity of rolling stock had quickly to be increased. From 5 November 1910 until 30 June 1911, the line carried 29,263,610 passengers.
Developments to the present
On 8 April 1911, the line was extended north to Pigalle. A subsequent three station extension to Jules Joffrin proved particularly difficult, as the route passed underneath the hill of Montmartre, which had long been quarried and mined for gypsum to make plaster of paris. During the tunnel's construction, numerous unknown quarries were discovered forcing a change in the line's route to avoid them. The two intermediate stations, Abbesses and Lamarck – Caulaincourt, are particularly deep : and below ground level, respectively, with Abbesses holding the network's depth record until Villejuif — Gustave Roussy opened in 2025, which itself will be dethroned by Saint-Maur–Créteil in 2026. The Nord-Sud engineers constructed arches underground to support the weight of the gypsum above. The extension was put into service on 30 October 1912.
Work on the final extension northwards began in September 1912, but the tunnel was not finished at the beginning of the First World War. Though lacking personnel, the construction work continued at a slower pace. On 23 August 1916, in the middle of the war, the line reached Porte de la Chapelle. This extension comprised three new stations, of which the final included three tracks and two central platforms, the central track being used by trains leaving or arriving. Line A intersected with Line 4 at Marcadet – Poissonniers, but no transfer had yet been set up.
On 1 January 1930, the CMP absorbed the Nord-Sud company, and line A became line 12 of the Métropolitain network on 27 March 1931, stealing the number 12 from an abandoned Fulgence Bienvenüe's complementary line from Porte d'Orleans to Porte d'Italie. Deeming the number of trains on the line insufficient, the CMP transferred four trains from their holding to augment the service.
The Nord-Sud company lines were powered through overhead cables, whereas the third rail system was in use on the CMP's network. With the ownership transition, the CMP standard was adopted on the lines 12 and 13. As a result, the overhead cables of the Nord-Sud were decommissioned in 1932, one year after those of line 13. Pantographs continued to be used in the Vaugirard workshops. To improve the lines' connection and carriages transfer, a new switch track was built in 1935 between the Montparnasse station of Line 12 and the Vavin station of line 4, still standing by to this day.
On 23 April 1930, a collision was caused by human error close to the Porte de Versailles station. A train moving north was stopped before a red signal between Porte de Versailles and Convention stations. A second hit it hard from behind, the driver passing two stop signals at full speed. Two people were killed, and numerous injuries were reported.
On 12 July 1928, the Seine general council decided to extend the Métropolitain by , taking it beyond the limit of Paris so that line A would serve the town of Issy-les-Moulineaux with two new stations. Work began in 1931 and led to the relocation of Porte de Versailles station further south, the new station being set up with two platforms staggered by . The platforms of the old station were removed and sidings were installed to house trains not in use. The extension comprised only two stations with long platforms, terminating at standard two track station, followed by a reversing area. On 24 March 1934, the southern extension to Mairie d'Issy was inaugurated, the same day as the line 1 extension east to Château de Vincennes.
On the night of 20 April 1944, during the Liberation of Paris, the freight station of Porte de la Chapelle and the RATP central workshop on rue Championnet were bombarded. The former, which is also the line's terminus, was severely damaged though hasty repairs returned it to service a few days later.
The line was equipped with a centralised control room in 1971. In 1977 the arrival of MF 67 rolling stock allowed the inauguration of Automatic Train Control on Line 12.