Until 2013, Line 4 was one of a few lines (the others being semi-circular lines 2 and 6, as well as Line 14) that had never been extended beyond the "gates" of Paris. During the 1920s, a preliminary extension towards la Vache-Noire was planned but never carried out. Since that time, no other extension plan was brought up. In 2008, nearly a century after its opening, construction of a one-station extension towards Mairie de Montrouge began. The new station officially opened to passengers on 23 March 2013, allowing one to travel from Montrouge to Clignancourt in 30 minutes. Mairie de Montrouge is a traditional two-tracked station.