Construction began on 3 November 1969. On 25 February 1978, the first Vienna U-Bahn route between Karlsplatz and Reumannplatz, the U1, went into operation. With twelve partial commissionings, the Vienna U-Bahn basic network was completed on 3 September 1982. Missing U5 Placeholder line numbers were awarded during initial planning of the Vienna subway network between 1966 and 1973. The designation U5 was used in initial and later plans, but ultimately none of the segments with its numeration were approved for construction. In early expansion variant plans, the U5 would have run between Meidlinger Hauptstraße and St. Marx using the already partially-tunneled southern belt route. It later referred to the current branch of the line U2 from Schottenring to the stadium, which was planned to connect to a new segment to Hernals. , there is no U5 line; today's U2 line consists of parts of the previously planned U2 and U5, which are connected by an arc between the stations Rathaus and Schottentor (this was originally planned only as an operating track and is still the narrowest curve in the Vienna subway network). The designation U3 was for a long time a gap in the network, but there was already in the construction of the basic network preliminary work. Thus, the entire tunnel tube of the U3 between Naglergasse / Graben and Stubentor was completed during construction of the U1 (at Stephansplatz), in order to avoid further excavation work in the area of the cathedral. Since 2003, several plans and internal working papers of Vienna have again been planning long-term plans for a U5 line, but only in early 2014 did they again make concrete efforts to actually realize the line. Finally, in March 2014, it was announced that the U5 line would be constructed in several stages of development as part of a U2 / U5 line cross. Starting at Karlsplatz, the new line will use the existing U2 section, with a new section to be built from Rathaus station northwards. In the first expansion step, however, the line will be run until 2023, for the time being, only up to one stop on Frankhplatz in the area of the old AKH. The further construction in the 17th district is planned; However, the construction costs must first be negotiated with the federal government. (See: Fifth stage of the subway network). The current U2 will then continue southwards in newly built