A newly-completed study by the local passenger watchdog Promoció del Transport Públic (PTP), which was presented at the Official Association of Civil Engineers of Catalonia, has concluded that the construction of a 3.6km line connecting the two halves of the current 29.1km tram network would be the best option to improve surface mobility across the city.

The link could be easily constructed as a segregated double-track line on the 50m-wide Diagonal Avenue, with a cost per kilometre starting at €15m for basic infrastructure and single-unit tram operation.

When compared with the current bus service on the corridor, a new tram line offers better use of surface capacity, a higher service speed, less pollution and noise and, crucially, savings of around €2m per year. The report estimates that the tram line would reduce bus traffic by 1.9-3.1 million bus-km per year, enabling vehicles to be redeployed to boost capacity on other routes .

The line would also improve connectivity throughout the Barcelona metropolitan area and the nine municipalities already served by the Tram: Badalona, Barcelona, l'Hospitalet, Esplugues, Cornellà, Sant Adrià de Besòs, Sant Feliu de Llobregat, Sant Joan Despí and Sant Just Desvern. As a result of the area covered by the project and the economies of scale, total annual ridership on the combined tram network could double to 50 million passengers, while utilisation of the tram fleet would also improve dramatically, increasing from 2.6 million-km per year to 4.6 million-km.

The feasibility of a new Diagonal tram line connecting the Trambaix (T1, T2 and T3) and the Trambesòs lines (T4, T5, T6, and T7) was previously assessed by the Metropolitan Transport Authority on its 2011-2020 masterplan, which estimated an internal rate of return of 44% for the project.

The Diagonal tram project was revived following May's local elections in Spain, and is now dependent on its approval by a majority vote by the city council, which is expected to provide most of the required funding, together with the support of the autonomous government, which oversees rail transport in Catalonia.