company, closed in 1928 and 1931 respectively. The Oerlikon to Schwamendingen and Seebach to Glattbrugg lines of the ZOS company, closed in 1931. The Uster to Langholz line of the UOeB company, closed in 1949. The Wetzikon to Meilen line of the WMB company, closed in 1950. In the city, the initial threat to the tram came from its perceived inflexibility and susceptibility to the growing traffic congestion in the city streets. One proposed solution was the conversion of the less busy lines to trolleybus routes, and the first step in this direction was the conversion of tram route 1, from Burgwies to Hardplatz (1954-1956). This was followed by the Farbhof to Schlieren section of route 2 (1956-8) which became a westward extension of the same trolleybus route. In practice, the trolleybus service struggled to cope with peak loadings and punctuality did not improve. No further conversions of tram routes to trolleybuses have taken place. Underground proposals In the 1950s, as well as proposing the conversion of less busy lines to trolleybus, plans were also made to place the busier lines in tunnel, in a form called the Tiefbahn. The recently delivered Swiss Standard trams were not seen as suitable for this, because they had doors on their tapered car ends that would not have aligned with the proposed underground station platforms. In order to overcome this limitation, several new designs of tram were introduced. The first design, known as the P16 or Karpfen, could not run on some existing routes, and only one batch of 15 motor tram and trailer pairs was built. A later design, which used articulation to avoid the problems of the P16, was eventually more successful and 126 vehicles were delivered by 1969. This car became popularly known as the Mirage. Despite the planning and new rolling stock, a referendum in 1962 rejected the Tiefbahn. However the proponents of going underground instead proposed a full scale metro, the Zürich U-Bahn system. This would have been standard gauge and electrified using a third rail, and hence incompatible with the tram system. The lines would have extended further into the suburbs and provided faster transit times than the tramways, which would have been curtailed so as not to compete with the U-Bahn. However this would have been at the expense of a coarser grained network, with much longer distances between U-Bahn stations than between the tram stops