they replaced. In 1973, the U-Bahn proposal too was rejected in a referendum, but not before several stretches of U-Bahn tunnel had been built. One section of the putative U-Bahn has since been adapted, as described below, for use by trams, whilst another now forms the terminus of the Uetliberg and Sihltal railway lines under the Hauptbahnhof. Extensions and a new model In 1976, the first tram extension since 1954 took place, with the extension of route 4 from Hardturm to Werdhölzli. Unlike the older lines, this extension was built mostly on reserved track, a precedent to be followed by most subsequent extensions. The Werdhölzli extension was followed in 1986 by the extension of routes 7 and 9 into new residential areas to the north-east of Zürich. This used one of the stretches of tunnel that had been built for the rejected U-Bahn, between Milchbuck and Schwamendingen. The tram route was extended through the tunnel before splitting at Schwamendingen to serve the area beyond, using new surface track. Because the tunnels and stations had been built with island platforms, whilst Zürich trams only have doors on their nearside, the section through the tunnel uses left-hand running. From 1976 onwards, the VBZ tram fleet was further updated, with the introduction of Zürich's variant of the Tram 2000 design used by several Swiss tram systems. Several sub-classes of the Tram 2000 were purchased, including articulated and non-articulated variants, and some without drivers cabs that could only operate in multiple with other cars. Eventually 171 of these vehicles were delivered, with the last of the class delivered in 1992. From the 1980s onwards, the system was increasingly acclaimed for its success in maintaining a high share of the modal split, and the Zürich model of transport provision was named after it. Beyond the tramway, the Zürich S-Bahn rail network was introduced to serve the region beyond the city boundaries, taking on some of the role that was originally planned for the U-Bahn. In 1990, the city's urban and regional transport were integrated by the introduction of the ZVV and its zone-based common fare structure. Low floors and the Glattalbahn In 2001, the VBZ took delivery of the first prototypes of a brand-new low-floor design of tram, known as the Cobra. Despite many teething problems with the prototypes, which were eventually extensively rebuilt, there are now 88 of these trams in service, with the