The Renfe Southern line was the most important result of the Souther Bypass, as its second phase. With a route of 3.2 kilometres, it connects the southern part of Bilbao with the left bank and the municipalities of the mining area, as well as connecting with the Adif Ancho Métrico line, the metro, the tram and the Bilbao Intermodal bus terminal.
Four new stations were built on the southern line (Zabalburu, Ametzola, Autonomía and San Mamés) and the Abando station was remodelled.
The work was technically very complex since it was necessary to demolish and rebuild two bridges over the tracks, in addition to carrying out the entire project without interrupting the railway service. Avant-garde technical solutions were incorporated such as the rigid catenary, specially designed for Renfe, representing a world record in line length for a voltage of 3,000 volts.