Barcelona Metro Diagonal line

The Diagonal line of the Barcelona Metro network was a proposed underground railway service in the Spanish city of Barcelona. The proposal was made by Transports Metropolitans de Barcelona (TMB) between 1989 and 1992. It would have been a line crossing the city's most important avenue, Avinguda Diagonal, from southwest to northeast, through central Barcelona. However, the project was finally abandoned.

As of 2009 the current project to link the two main modern tram systems (Trambaix and Trambesòs) through Avinguda Diagonal may be considered its successor, even though the exact location of the stations and the nature of the tram system to be used have not been finalized.