Normally very crowded, Line A is estimated to transport nearly half a million people daily, though because of restructuring work on the line, trains are currently replaced by buses after the departure at 22:00.
Work began in 1964 in the Tuscolana area and immediately ran into unexpected delays and difficulties, an example of which was the disruption caused to traffic in the south-east of Rome by the Cut and cover method of digging. The work was suspended and only resumed 5 later years with tunnelling machines which, although helping to ease traffic problems, caused vibration damage to buildings.
Archaeological discoveries were frequent during the work, in particular in the area of the Piazza della Repubblica, and required changes to the planning. The uncovered remains were put on show in protective glass display cases in Repubblica station. The tunnelling work and connected archaeological discoveries were portrayed in the Federico Fellini Roma.
The line entered service in 1980, from Anagnina to Ottaviano and took the name of Line A, while the existing Termini-Laurentina line was called Line B. In the early 1990s work began on an extension to Line A from Ottaviano to Battistini, which opened during 1999 and 2000.
In January 2005, the 1980 rolling stock began to be replaced by new design, air-conditioned S/300 trains, built by the Spanish company Caf (Construcciones y Auxiliar de Ferrocarriles).