The line was constructed by the Compañía Hispano Argentina de Obras Públicas y Finanzas (CHADOPyF, Hispanic-Argentine Company for Public Works and Finances), headed by a Spanish nobleman, Rafael Benjumea, the Count of
Guadalhorce. This company took on the task because the Great Depression of the 1930s meant that the
Anglo-Argentine Tramways Company (which owned most of the then vast tram network in the city) pulled out of its construction. The line was subsequently opened in 1934 and extended from the present terminus at Constitución to
Diagonal Norte in the centre of the city. The stretch from Diagonal Norte to its present terminus at Retiro was opened in 1936 and marked the last expansion of the line, which remains the only line not to be extended since then. On opening, the stations were decorated with murals depicting rural
Spain created by Argentine artists Martín S. Noel and Manuel Escasani, as well as some stations reflecting
Moorish influences and were painted with gold powder. It had advanced technology for the time, such as
Automatic Train Stop (ATS). With the extensions of Line E and Line H, they now meet Line C at Retiro, where all three terminate.
Siemens has modernised the signalling systems on the line, at a cost of $18 million which included the use of
Communications-based train control (CBTC) in order to improve the line's frequency and potentially open up the possibility of automatic trains in the future. The work was completed towards the end of 2016. Aside from the signalling system, the line has had its stations refurbished, its rolling stock refurbished and ventilation improved, in part to compensate from the extra heat associated with the air conditioning units added to the trains. Major works took place at
Constitución railway station to provide better transfers between the
Roca Line, Line C and
Metrobus Sur in the area, which includes the construction of a new underground annex to the station. ==Rolling stock==