The history of the SZU dates back to two separate companies, which built the two railways that now make up the SZU. The first of these companies was the Uetlibergbahn-Gesellschaft, which opened its line from Bahnhof Selnau in Zurich to the summit of the Uetliberg mountain in 1875. This was followed in 1892 by the Sihltalbahn company, which opened a line from Bahnhof Selnau to Sihlwald. In 1897 this latter line was extended to Sihlbrugg and a connection with the Thalwil to Zug line of the Swiss Northeastern Railway. In 1920, the Uetlibergbahn-Gesellschaft became bankrupt and was liquidated. Two years later the Uetlibergbahn was taken over by the Bahngesellschaft Zürich–Uetliberg. In 1923 the Uetlibergbahn was electrified using the direct current system, whilst the following year the Sihltalbahn was electrified using alternating current. In 1932 the SITB took over the management of the BZUe, but the two companies remained in existence until 1973, when they were merged to form the SZU. In the meantime, in 1954, the SITB had taken over the management of the Adliswil-Felsenegg cable car. In 1990, the two lines were extended from their previous joint terminus at Bahnhof Selnau to a terminus at Zürich HB. This extension involved the construction of a new rail tunnel from Selnau to Zurich HB, and a new underground intermediate station adjacent to the former terminus. The underground platforms used at Zurich HB were already in existence, having been built prior to 1973 for a U-Bahn scheme that was ultimately rejected by voters. Once the new extension had been opened, the former terminus at Selnau was redeveloped and little evidence of it is now visible. In 1995, the company took over responsibility for bus services in parts of the district of Horgen.
Operation
Rail network
The SZU continues to operate the original Uetlibergbahn and Sihltalbahn lines, although there is currently no regular service over the outermost section of the Sihltalbahn line between Sihlwald and Sihlbrugg. The two lines share a common double-track section between Zürich Giesshübel and Zürich Hauptbahnhof, with the final approach in tunnel, partly under the Sihl river. A dedicated pair of underground platforms are used at the Hauptbahnhof with no rail connection to the rest of the station. Also operated is a connecting line from Giesshübel to the Swiss Federal Railways at Zürich Wiedikon, although this is normally only used for freight traffic. In all, the SZU network measures : km as part of the Sihltalbahn and of the Uetlibergbahn. The passenger services on the two lines now form part of the S-Bahn Zürich, with the service over the Sihltalbahn to Sihlwald branded as the S4 and the Uetlibergbahn branded as the S10. Both railway lines are constructed to. They are unusual in that, whilst the Sihltalbahn is electrified using the standard Swiss mainline system of overhead lines at 15 kV 16.7 HzAC, the Uetlibergbahn is electrified using overhead lines at 1200 VDC. In order to avoid conflict on the common section, the Uetlibergbahn uses an overhead line offset from the centre of the track, and its cars are equipped with specially designed pantographs to collect from this.
