It originally contained only 8 stops connecting Cuatro Caminos in the north to the city center at Puerta del Sol.

It originally ran from Cuatro Caminos and Sol, containing 6 intermediate stops: Red de San Luis (now Gran Vía), Hospicio (now Tribunal), Bilbao, Chamberí, Martinez Campos (now Iglesia), and Ríos Rosas.

[3] The original line was extended from Sol to Atocha in 1921, then further to Puente de Vallecas in 1923.

The old Chamberí station, however, was reopened to the public in 2008 as part of the Madrid Metro's Platform 0 project, now functioning primarily as a historic exhibition.

On 16 May 2007, the line was extended south from Congosto to Valdecarros with two intermediate stations.

[8][9] Since the summer of 2007, the line uses CAF class 2000A rolling stock trains.