for Milan. The Piano Regolatore Generale Comunale (English: General Municipal Plan) (PRGC) of 1953 called for the complete elimination of trams from the city centre, and their replacement with an underground Metro network (a proposal that had already been made before the war). Implementation of this plan began rapidly. In 1957, work started on the excavation of the first Metro line, leading to the removal of tramway lines from some critical transport axes (Corso Buenos Aires, Corso Vittorio Emanuele, Via Dante, etc.). Closed sections of tramway were not later reinstated, but replaced by new bus routes (Viale Liguria, Via Canonica). On the other hand, two tramway extensions were opened, to serve the new suburbs of Taliedo (1964) and Gratosoglio (1969). The 1950s and 1960s also saw the closure of almost all of Milan's interurban tramways, and their replacement by bus routes. A further sharp contraction of the tram network took place on 9 March 1970. In conjunction with the introduction of time based ticketing (in place of tickets for single trips), the network was generally reorganised, with duplicated lines being eliminated. Two years later, trams also disappeared from Corso Garibaldi, to make way for the construction site of Metro line 2. Nevertheless, the Milan tram network, although reduced, was still modernized. In 1971, new high capacity series 4800 Jumbotrams entered service. Their construction had involved the joining together of body shells of older trams. Until 1972, all trams carried conductors, but these were phased out between 1972 and 1979, except on the interurban lines, as ATM gradually converted its trams to one-man operation. Running parallel to that programme was ATM's conversion of its urban tram fleet from trolley pole current collection to pantographs, requiring modification of the overhead trolley wiring, which was spread out over several years for reasons of cost and practicality. The conversion to one-man operation and use of pantographs began in August 1972, with a single route (then route 2), and was completed in early 1979 with route 23. The two interurban tram lines to Desio (with branch to Milanino) and Limbiate continued to use trolley-pole-equipped cars until autumn 1983 and October 1986, respectively. In 1976, the series 4900 trams were ordered, in anticipation of the (never realized) transformation of the trolleybus circular route 90/91 into a light rail service. In 1981, the Via Larga tramway was opened, to reduce transit times in the city centre. In