The network of the Stadtbahn covers much of Stuttgart and also reaches the neighbouring towns of Remseck am Neckar, Fellbach, Ostfildern, Leinfelden-Echterdingen and Gerlingen (clockwise).

[2] As of 2024[update], the Stuttgart Stadtbahn system consists of: The U6, U7 and U12 lines run Monday to Saturday during the day as double sets as 80-metre long trains.

[7] The U9 was routed via Charlottenplatz to Heslach Vogelrain and supplemented by the additional line U29, which connected to the Hauptbahnhof, which can only be reached from the north, with the actual terminus of the U9 at Vogelsang (in peak hours in Botnang).

[9] With the timetable change on 9 December 2017, as part of the closing of the gap on the U12 between Hallschlag and Wagrainäcker, the U14 was withdrawn to Mühlhausen and the U12 was operated with double sets to its previous terminus in Remseck-Neckargröningen.

In 1961, the local council decided to move the rail network in the city centre underground, to switch to new, more powerful vehicles and to separate it from motor traffic in the outer areas as far as possible.

In the mid-1970s, the introduction of an underground railway was discussed, which would take over the new tunnels, until the decision was made in 1976 to gradually replace trams with a more efficient light rail system.

[22][23] The main criteria for the expansion of the Stadtbahn system were the conversion to standard gauge and the development of a bidirectional vehicle so that the turning loops at the end of the line could be eliminated.

After the opening of the Waldau tunnel at the end of May 1998, the Stadtbahn line U7 from Killesberg no longer ran to Degerloch Albstraße, but to the new Ruhbank (Fernsehturm) stop.

The new peak hour line U8, established at the instigation of the town of Ostfildern, has since been running from Vaihingen to Nellingen via the Weinsteige-Waldau triangle, which was previously only used for commercial purposes.

It was opened on 16 July 2005 with a 10–15 meter deep, 1.1 km (0.68 mi) long tunnel built by mining under the main cemetery (Hauptfriedhof).

A three-rail track was laid on the two converted sections of the southern branch, which enables vintage car trips with meter-gauge vehicles from the tram museum in Bad Cannstatt via the Nordbahnhof to Ruhbank.

To date, 14 more tunnels have been built after those mentioned above: Neue Weinsteige (opened in 1987), Degerloch (1990), Feuerbach Wiener Straße (1990), Weilimdorf (1992), Killesberg (1993), under the Botnanger Sattel (1994), Gerlingen (1997), Waldau (1998), Sillenbuch (1999), Ruit (2000), Steinhaldenfeld (2005), Fasanenhof (2010), Zuffenhausen (2011) and Hallschlag (2017).

In preparation for the 2006 FIFA World Cup, all stops (except Berliner Platz (Hohe Straße)) along the inner-city loop were expanded so that double sets with a length of 80 metres could be used on the event line U11.

Since the mid-2010s, the SSB has been gradually carrying out major renovations on older sections of the line, with the tracks and operating facilities mostly being completely replaced and some raised platforms also being renewed.

[28] The Rosenstein Bridge, which connects Cannstatt-Mitte with the Neckarvorstadt, was closed at short notice at the beginning of May 2022 due to extensive problems with the reinforced concrete structure in static load testing.

[29] Until a replacement is built in an unspecified period of time, the trams on lines U13 and U16, which regularly run over the bridge, must be diverted via the connecting curve at the Leuze junction between Wilhelma and Mercedesstraße stops.

Since 2017, the U12 Stadtbahn line has been running with double sets from Dürrlewang in the south via the city centre, through Europaviertel, the Nordbahnhofviertel and Hallschlag down into the Neckar valley to Remseck in the north.

The reason for the change in planning was overly high traffic forecasts of up to 18,000 vehicles per day for Rosensteinstrasse, as well as concerns at the political level about a premature closure of the existing branch on Friedhofstrasse.

Due to the excavation pit for the southeastern tunnel portal, the line between Wilhelma and Mineralbäder or Mercedesstrasse ran on a single track over a temporary bridge and had to be relocated again after its completion.

[46] In late summer 2013, the negotiators of the cities of Stuttgart and Leinfelden-Echterdingen, the state government, the SSB and the district of Esslingen agreed on the financing.

[56] As part of this project, the remaining southern branch of the U5 from Möhringen Freibad will also be completely rebuilt during a two-month closure from July 2024 to September 2024.

Construction of the first, particularly busy, section of the line began in March 2023 with the eight stops on the northern branch (Bad Cannstatt – Uff-Kirchhof – Fellbach Schwabenlandhalle).

[64] Their reconstruction is considered problematic in terms of urban development, as a simple extension of the platform in both directions (east and west) would interrupt important pedestrian and cycle paths as well as sight lines.

The Pflugmühle curve between the SSB-Zentrum and Rohrer Weg stops, which was completed in 2022, enables a direct route from Vaihingen and Dürrlewang to Flughafen/Messe (Airport/Trade Fair) and to Leinfelden.

Of all the Stadtbahn projects, the fourth depot has the highest priority because the three existing ones can no longer accommodate additional sets and without them no improvements to the service would be possible, regardless of whether it be increased frequency, more 80 m (260 ft)-long trains or line extensions.

In 2023, there was talk of an increase in costs to around €440m, of which €230m was for the depot, due in part to an outdoor geothermal plant, wastewater heat exchangers and an energy centre for Hausen.

[83][84] The connection of the district of Hausen to the Stadtbahn network was originally supposed to be achieved by an approximately 3.1 km (1.9 mi)-long extension of the U13, starting from the then terminus at Giebel, flush with the street through Rappachstraße, along Gerlinger Straße and Hausenring to the northern development boundary of Hausen (Straße Beim Fasanengarten) and further across open fields to the S-Bahn station at Weilimdorf.

The route that has been kept free for this purpose can be clearly seen in the gap between the student residences and the retail supermarket as well as the Probststraße bridge over the Stadtbahn.

The resulting direct Killesberg–Plieningen line running every 20 minutes (in the gap between the U5 service between Killesberg and Leinfelden) was temporarily referred to internally as U5b,[85][86] since 2022 it has been officially known as U25.

[88] The resistance of Stuttgart municipal representatives, especially for the Killesberg, where a more frequent 10-minute cycle is demanded, was countered with a planned partial commissioning of the U25 on the Killesberg–Möhringen Bf section as early as the end of 2025 and an extension to Plieningen via the new curve in 2027.