Roma–Civita Castellana–Viterbo line (also called Roma Nord railway) began life as a narrow-gauge tram running from Piazza della Libertà in Rome to Civita Castellana. However, the next stretch of the line, to Viterbo, was built as a railway and over the years the tram section was converted into a railway as well, a process which concluded with the moving of the Roman terminus from the street-level terminus at Piazza della Libertà across the river to a new underground station in Piazzale Flaminio, beside the subsequently constructed Line A station, after World War II. The line is operated in two modes: as an urban service from Piazzale Flaminio to Montebello, and as a suburban service from Piazzale Flaminio to Viterbo. The urban service operates with a frequency of about one train every ten minutes, while the suburban service operates considerably less frequently, with less than a third of the trains making the full two-and-a-half–hour journey from Rome to Viterbo. Future expansions The Metro system is currently expanding: An extension of line A from Battistini to Torrevecchia towards the west of the city is planned; a track of 2 kilometres (1.2 mi) with 2 stations. An extension of line B is also planned. A track of 2.8 kilometres (1.7 mi) with 2 stations should be realized from Rebibbia (Rome Metro) to Torraccia/Casal Monastero, towards the east of Rome. The extension of the line B1 is extending from Jonio to Bufalotta with 3.8 kilometers with 3 stations. The extension of line C towards the centre of Rome is under construction. From San Giovanni (interchange station with line A) to Grottarossa (Rome Metro) , the track is 3.6 kilometres (2.2 mi) long with an intermediate station near San Giovanni Addolorata Hospital, Amba Aradam. New lines There are two proposed new lines. There are no timelines yet for their construction. Line D would be a completely new line linking Ojetti and Agricoltura; while line E would see the existing Rome–Lido railway extended from its Porta S. Paolo terminus to Piramide using the existing Line B infrastructure. Signalling Signalling of the Rome Metro guarantees trains' safe and correct movements. Line A Line A uses an evolution of the RS4 Codici, a classical block system of the Italian railway. Since its creation the signalling offers to the conductor advice about the speed limit and the freedom of the way. Line B Until 1990, Line B used a