The €401m extension runs entirely underground with three new stations, including an interchange with Renfe Rodalies lines R4 and R12 at Terrassa-Estació del Nord.
Preliminary works on the line started in October 2003 but construction suffered several delays due to project changes and a shortage of funds. Contractors involved in the project included Spanish Construction companies Copisa, OHL and FCC, along with Siemens Rail Automation.
The line runs in 3.1km twin-bore tunnels, which were excavated using earth pressure balance TBMs, and the project included the construction of a new depot at Terrassa Nacions Unides station.
The extension will be served by up to eight trains per hour during the peak and four trains per hour off-peak, using FGC's new fleet of Alstom/CAF class 112 and 113 EMUs. Demand for the new section is forecast at 5.5 million passengers per year, a 7% increase in FGC ridership, which totalled 77 million in 2014.
Work is also nearing completion on another addition to FGC's Barcelona - Vallés line with the 4,4km five-station extension of Line S2 beneath Sabadell due to open next year.
In addition, FGC is also planning a 4.1km extension of its Llobregat - Anoia metre-gauge suburban line from its current Barcelona terminus at Plaça de Espanya beneath the city's central Eixample district to Gràcia.