Plaça de Catalunya station serves as the Barcelona terminus of the line, where almost all its trains either start or terminate.

Its main route splits in two in Sant Cugat del Vallès, forming two major branches to Sabadell and Terrassa.

The origins of the line date from 1863, when a privately owned railway from Barcelona to the then-separated town of Sarrià was opened.

In 1912, Catalan engineer Carles Emili Montañès created the company Ferrocarriles de Cataluña (FCC) in order to take control of the line and extend it northwards.

Currently, the most prominent intervention on the line is the extension of the Terrassa and Sabadell branches through the construction of a route underneath the two cities; the former was completed in 2015, whilst the latter did so in 2017.

Construction of the subway line started in late 2006 and was due to be finished by 2011 or 2012, depending on which project the FGC finally decided on.

[4] However, work was temporarily halted due to the 2008–2014 Spanish financial crisis,[5] but later resumed and the first phase opened in two stages in September 2016 and July 2017.

The terminus of the current general FGC line, a station called Sabadell – Rambla, will disappear as it will be replaced by this network.

It could be more accurately defined as an underground addition to the already existing FGC commuter train network in the city, located in the metropolitan area of Barcelona.

[9] Renfe itself is going to build another station in the city: Terrassa Est – Passeig del Camp[11] Parts of the Vallès line operated by FGC are actually known under the collective name of Metro del Vallès since 1996, which will include the Terrassa service and the extension of S2 which will become Sabadell Metro in the almost adjacent municipality of Sabadell, which is already under construction.

[12] The Barcelona–Vallès Line has its start point at Plaça de Catalunya station, which is located directly underneath El Triangle shopping mall, on the southwesternmost side of Plaça de Catalunya ("Catalonia Square"), a major public transport hub in the Barcelona district of Eixample.

The line then continues northwestwards in an underground route along Balmes Street until its intersection with Diagonal Avenue, from this point on running underneath Via Augusta.

At this point, the line runs on a succession of alternated open-cut and underground sections before eventually crossing the Vallvidrera neighborhood through a 1.6-kilometre-long (0.99 mi) tunnel in a northwest direction.

[16] It then continues going across Collserola's central area above ground, running parallel to C-16 motorway up to Valldoreix station in Sant Cugat del Vallès, passing through Les Planes and La Floresta neighborhoods.

[15] On the one hand, the stretch to Terrassa continues moving away from Sant Cugat del Vallès in a northwesterly direction towards Rubí, where the line's main depot and operations centre is located.

It goes along the western limits of Rubí city centre, describing a straight route northwards to the Les Fonts neighbourhood.

[17] On the other hand, the stretch to Sabadell curves northeasterly as it approaches the end of its route through Sant Cugat del Vallès.

[19] The signaling and security systems used on the line are centralized traffic control (CTC) and automatic train protection (ATP).

The ICC is also in charge of supervising and regulating the facilities found in the line's stations such as elevators, escalators and ticket vending machines, among other functions.

[27][28][29] Continuous overnight rapid transit services had previously been offered on Saturdays, though they were suspended to reduce operational costs in January 2012.

There are a total of 66 electrical multiple units, all of which are disabled-accessible, air-conditioned and equipped with the automatic train protection (ATP) security system.

[48] The 113 and 114 Series are identical in both their technical features and design, though they differ in the number of cars and present some modifications on the seat layout.

[49] In December 2017, FGC ordered 15 new four-car EMUs, designated the 115 Series, from Stadler Rail's ex-MACOSA facility with entry into service planned for 2019.