Lines are designated by a number and the prefix "U" (for U-Bahn) and identified on station signage and related literature by a colour.

[citation needed] Ticketing for the network is integrated under the Verkehrsverbund Ost-Region [de] (VOR) along with other means of public transport in Vienna, including trams and buses.

[citation needed] The concession request of the engineer Heinrich Sichrowsky dates from 1844 with the idea of a pneumatic, or atmospheric, railway based on the system of Medhurst and Clegg.

This would have created a connection between the central station and the market halls, while at the same time the gas-lit tunnels were to serve as warehouses for food.

The conception of the British engineers James Bunton and Joseph Fogerty won out, since their plans, approved in 1881, included trains to be run in tunnels, in open incisions, and on elevated tracks.

[citation needed] In 1883, the project of an "electric secondary railway" of the company Siemens & Halske planned for a small profile rail system with three lines.

The first system to be constructed was a four-line Stadtbahn railway network (which had been planned to have three main and three local lines) using steam trains.

Starting in 1910, plans were considered for an underground system, but were interrupted by the First World War, which also necessitated closing the Stadtbahn to civilian use.

Both in 1937 and after the Anschluß, when Vienna became the largest city by surface area in Nazi Germany, ambitious plans for a U-Bahn, and a new central railway station, were discussed.

But it was not until the late 1960s, when the Stadtbahn and the Schnellbahn were no longer able to adequately serve the ever-increasing public traffic, that the decision to build a new network was taken.

Construction began on 3 November 1969[4][11] on and under Karlsplatz, where three lines of the basic network were to meet, and where central control of the U-Bahn was located.

In early expansion variant plans, the U5 would have run between Meidlinger Hauptstraße and St. Marx using the already partially-tunneled southern belt route.

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.

However, due to insufficient urbanization, this project was not found to be meaningful and was never planned, as it would be possible to transport almost the same number of people by means of a much cheaper tram line, which is the replacement of tram line 26 east of Wagramer Straße, from there to the Ziegelhofstraße six stops further on its own track body, by the Gewerbepark Stadlau to the subway station Hausfeldstraße on the northern edge of Aspern was also reached.

After completion of the basic network, the Vienna subway system was extended in 1989 to the line U6 with the route Heiligenstadt-Philadelphiabrücke (10.6 km (6 mi 47 ch)).

For the first time in Europe, a U-Bahn project had to undergo a costly and lengthy environmental impact assessment, as the U2 extension showed a length of more than 10 km (6.2 mi)).

[2] This expansion phase involved: On 19 October 2001, the groundbreaking ceremony for the extension of U1 was held, for which the two districts had been waiting for 20 years.

The change to the original plans was thought to be due cost issues or the incomplete development of the area surrounding Rothneusiedl.

This extension was ultimately opened to the public on 2 September 2017, thereby expanding the Vienna metro network by 4.6 kilometres (2 mi 69 ch) and 5 stations.

The originally planned southern extension of U2 to Gudrunstraße indefinitely delayed for financial reasons and since the need suggested is no longer there.

The line U2 coming from Seestadt and Schottentor will receive a new south branch, leading to the S-Bahn station Matzleinsdorfer Platz.

The remaining route of the U2 between Karlsplatz and Universitätsstraße will be taken over by a newly created U5 line, which will be supplemented by the station Frankhplatz (Altes AKH) for the time being.

The U6 has one class of train, the T/T1 type (introduced in 1993), the older E6/C6 having been retired in 2008 and now mostly operating in Utrecht in the Netherlands and Kraków in Poland, with a single set being preserved at Vienna's tramway museum ("Remise").

Additionally, the trains are gradually being retrofitted with plastic seats, video surveillance, and warning lights to signal door closures.

The newer Type V trains also have yellow handrails instead of gray-red ones, improved interior displays, and warning lights to indicate door closures.

At both ends of the train, there are multipurpose compartments with four folding seats each, as well as automatically extending ramps at every station to close the platform gap.

To minimize station dwell time and prevent obstructions by passengers, the doors use sensitive sensor edges as anti-trap mechanisms instead of light barriers.

The trains are equipped with extensive safety technology, including fire detectors in the roof areas, temperature sensors, and dry extinguishing pipes on the undercarriage.

These vehicles feature air conditioning, video surveillance, and electronic interior and exterior displays, along with an updated design.

At the end of 2009, Type T cars began receiving upgrades, including electronic interior and exterior displays and video surveillance to enhance passenger and staff safety while reducing vandalism.