Line 9 is to be the longest metro line in Europe.

At 48 km Line 9 will be the longest metro line anywhere in Europe and is to have 52 stations. It will cross the city of Barcelona and provide a superb communication link for neighbourhoods with the greatest demand for public transport in the city. The line will also connect with the logistics activities areas in the port and the airport.

The railway tunnels have been largely dug with EPB (Earth Pressure Balance) tunnel boring machines with diameters of 9 or 12 m depending on whether the sections are designed to accommodate single or double deck trains. The tunnel network was rounded off with sections built using screen tunnels and conventional excavation methods.

Especially significant is the branch in Zona Franca which consists of 2,700 m of line on viaducts and 4 surface stations, as drilling is impossible in this industrial area bordering the sea.

The method chosen for constructing the stations was digging cylindrical shafts 28 m in diameter and up to 70 m deep.

The construction of the platforms involves the superposition of two levels both located inside the tunnel, and instead of the traditional metro line with trains at the same level there are two independent levels separated by a structural slab.

The special features of the area meant that fewer underground stations could be built using the conventional method of screened enclosures.