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The Stuttgart Stadtbahn succeeded a tram network that previously dominated urban transport. 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.^[1]

The first tunnel stop was built in 1966 at Charlottenplatz in coordination with the reconstruction of the intersection of federal highways B 14 and B 27, where the routes of the central valley longitudinal (roughly north-south) lines (at that time parallel to the B 14) intersect with the valley transverse lines (parallel to the B 27). A tunnel between Neckartor and Marienplatz (running close to the B 14) was completed in 1972. All tunnel routes and stops were already built with a clearance profile suitable for the planned standard gauge vehicles and the track system was laid out with a third rail for standard gauge. The new vehicles were to be high-floor and the stops were given high platforms.

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. By 1978, Königstrasse and the Hauptbahnhof as well as the sections to the then Universitat (the former site of the university, now Börsenplatz) and Türlenstrasse (now Stadtbibliothek) stations were tunnelled, followed by the Rotebühlplatz stop by 1984, the shell of which was already built in 1978 as part of the S-Bahn construction, and the Siemensstrasse tunnel between Pragsattel and Feuerbach station.

The Stadtbahn network is designed in such a way that it could be converted to a U-Bahn at any time if the financial means were available. Contrary to popular belief, the U in the line name does not stand for underground railway, but for unabhängig—independent (from road traffic).^[2][3]

The main criteria for the expansion of the light rail 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. In addition, barrier-free access to the trains should be possible without having to climb steps. The DT 8 light rail vehicle, specially developed for Stuttgart , took these requirements into account.

Both systems had to be operated alongside each other for a longer period of time with interim solutions. The routes intended for light rail operations were converted to a three-rail track . The third rail on the inside was later gradually removed where meter-gauge trams were no longer running. Some sections will remain permanently as a three-rail track to enable museum tours of the Stuttgart Historical Trams . The museum tours lead from the Bad Cannstatt Tram Museum to the city center or to the Ruhbank/Fernsehturm stop.

A further challenge was the different platform heights. With the old trams, boarding was only possible from the street or from an underground platform; with the new light rail vehicle, boarding was possible either via an underground platform or, preferably barrier-free, via an elevated platform. The older type of light rail vehicles (DT 8.4, DT 8.5, DT 8.7) are equipped with so-called folding steps , which can be folded out at the underground platform. While both types of vehicle were still in operation, half of the platforms were converted to high platforms. As soon as the trams were no longer running, the other half could be converted. As part of the program to make the light rail barrier-free , all stops were equipped with high platforms and, where necessary, elevators by December 12, 2010. The low platforms will remain at the stops for the museum lines 21 and 23.

From 25 July 1983, three pre-series vehicles of the Stuttgart Stadtbahn railcar DT 8 ( DT 8.1 to DT 8.3 ) were used in trial operation on the Plieningen–Möhringen section of line 3.On September 28, 1985, line 3 from Plieningen to Vaihingen was the first line to be completely converted to a light rail system.This was followed by lines 1 and 14 on April 19 and July 12, 1986, which were the first times that trams ran through the city center. In 1988, the Volksfest line (later U11) to Cannstatter Wasen was served by trams for the first time, in 1989 line 9 was converted, and in 1990 two new tram lines, the valley crossing lines U5 (Freiberg–Leinfelden) and U6 (Feuerbach–Möhringen), were put into operation simultaneously for the first time.

After the U6 line was extended to Giebel following the completion of the Weilimdorf tunnel in 1992, it ran for the first time to the city limits of Gerlingen ( Gerlingen Siedlung stop ) in 1993. In 1997, the line was extended in the tunnel beneath the town to the town center. The U7 (Killesberg Messe–Degerloch), which was introduced for the International Horticultural Exhibition in 1993, was the first line to run in double traction in the spring of 1993.

In 1994, the tram reached Botnang with the converted line 4. In 1997 and 1998, line 13 was converted (first the section Feuerbach–Schlotterbeckstraße, then the section Schlotterbeckstraße–Hedelfingen). After the opening of the Waldau tunnel at the end of May 1998, the tram line U7 from Killesberg no longer ran to Degerloch Albstraße, but to the new stop Ruhbank (TV tower). In September 1998, an extension to Heumaden via Sillenbuch followed. In 1999, the next generation of tram vehicles arrived with the DT 8.10.

In the same year, the extension of the U14 from Mühlhausen to Remseck-Neckargröningen was the first network expansion since the beginning of the light rail era, compared to the tram network. One year later, on September 9, 2000, the 6.3-kilometer-long new line between Heumaden and Nellingen went into operation. The new HVZ line U8, established at the instigation of the city of Ostfildern, has since been running from Vaihingen to Nellingen via the Weinsteige-Waldau triangle, which was previously only used for commercial purposes. In addition, the now-discontinued event lines U16 (replaced by U19 in 2006), U17 and U18 began operating (see also the "Special lines" section).

On 22 June 2002, line 2 was the penultimate line to be converted to light rail operation and at the same time received a new line beyond the previous terminus Obere Ziegelei to the main cemetery . On 14 December of the same year, the previously remaining section Berliner Platz – Hölderlinplatz followed. In the meantime, the U2 ran clockwise through the inner city loop. The late conversion of the U2 is probably due to the complicated route, which made it difficult to create a separate track. A large part of the route is still flush with the street, and in one place it was even necessary to create a track loop. It was not until 1999 that the expansion and simultaneous extension to Neugereut was decided, which was opened on 16 July 2005 with a 10–15 meter deep, 1.1 kilometer long tunnel built by mining under the main cemetery.^[4]

From September 2005, the last meter-gauge section of line 15 was converted to light rail operation. First, the southern branch from Olgaeck to Ruhbank and the section through the Nordbahnhofviertel were completed while maintaining operational operations. In December 2007, the U15 between Ruhbank and Zuffenhausen Kelterplatz went into operation. A three-rail track was laid on the two converted sections of the southern branch, which enables vintage car rides with meter-gauge vehicles from the tram museum in Bad Cannstatt via the Nordbahnhof to Ruhbank. From the beginning of 2008 to the end of 2011, the northern branch from Zuffenhausen to Stammheim was converted; the most challenging part of this consists of a tunnel under Unterländer Straße.On the southern branch between Olgaeck and Ruhbank, the city railway - like the tram before it - manages a steep gradient of up to 8.5%. This makes this section the steepest standard gauge friction track in Europe used by local public transport. Of this, 65 metres in altitude are accounted for by the distance between Olgaeck (260 m) and Heidehofstrasse (325 m), i.e. a route length of around 800 m. This gives an average gradient of 8.1%, with the maximum being 8.5% on Alexanderstrasse.

Since December 10, 2011, the outer branch of the U15 through the new tunnel in Zuffenhausen to Stammheim has been in operation. The Stammheim Rathaus and Stammheim stops were merged. Due to the route change between the Kirchtalstraße and Salzwiesenstraße stops, the Zahn-Nopper-Straße stop is no longer available . The Kirchtalstraße stop is the only tunnel stop where trains cannot stop in double traction. A connection between the Stadtbahn and S-Bahn in Zuffenhausen, which was envisaged in earlier plans, was not implemented because the tunnel section of the Stadtbahn passes under the railway line to Ludwigsburg a considerable distance from the Zuffenhausen S-Bahn station.

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). The tunnel between the main station and the city library was replaced in 2016-2017 by a new building in a different location with an additional branch towards the Europaviertel. The total length of the underground routes is around 26 kilometers.

With the delivery of the last DT 8.11 tram cars, the SSB owned 164 tram vehicles in 2005. At the timetable change in 2005/06, the U5 line was extended in the north by 650 meters to Mönchfeld . At the timetable change in 2010/11, extensive changes were made to the line network and the U6 received a new southern section to the Fasanenhof-Ost industrial estate (see Network 2011 ). From 2013, the extensive new construction of the U12 line followed.

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 traction with a length of 80 metres could be used on the event line U11.^[4]

As the new long-distance rail tunnel as part of Stuttgart 21 is being built in the area of ​​the new main station under Heilbronner Strasse at almost the same level as the current main station–Pragsattel light rail tunnel, the light rail route had to be lowered and moved. Parallel to the existing light rail tunnel, two tunnel tubes were initially excavated in Kriegsberg. The tunnel leading into the city was connected between 18 and 20 November 2016. [ 57 ] The tunnel leading out of the city was connected between 7 and 9 April 2017. [ 58 ] Since then, the old tunnel is to be filled in and the new long-distance rail tunnel is to be built over the light rail tunnel. This measure is being financed from the funds of the Stuttgart 21 project, including reimbursement of costs for possible operational disruptions.

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 elevated platforms also being renewed. As part of this, the last three-rail tracks outside the routes of the vintage lines are gradually being removed.^[5]

As part of the closure between the main station and the Staatsgalerie, the remaining sections of the existing tunnel stubs in the direction of the former Staatsgalerie stop will be converted into a turning facility in order to allow additional capacity on the inner-city loop. [ 60 ]

The Rosenstein Bridge, which connects Cannstatt-Mitte with the Neckarvorstadt, was closed at short notice at the beginning of May 2022 due to extensive static problems with the reinforced concrete structure. [ 61 ] Until a replacement is built in an unspecified period of time (see below), the trams on lines U13 and U16, which regularly run over the bridge, must be diverted via the connecting curve at the Leuzeknoten and the Wilhelma and Mercedesstraße stops . The Neckar crossing thus takes place via the König-Karls-Brücke, which is about 600 m upstream . In connection with a previously planned track renewal in Pragstraße, shortly after the closure, the switches in the direction of the Rosenstein Bridge were cut at the stop of the same name and replaced by simple track curves in the direction of Neckartalstraße. In addition, the tracks on the bridge were dismantled in May 2023.^[6] The dismantling of the now superfluous tracks in the crossing towards the bridge followed in autumn 2023.

The entire route through Badstraße, including the Bad Cannstatt Wilhelmsplatz (Badstraße) stop , will therefore become obsolete and will be closed for an indefinite period of time.^[7] The tram route at the stop has meanwhile been tarred over and converted into a road access for Badstraße.^[8]

The plan is to demolish the bridge by summer 2024 and then rebuild it as a wider arched bridge by around 2031.^[9] The Rosensteinbrücke tram stop, which was previously located on Pragstrasse, is to be relocated to the new bridge to provide better connections to Cannstatt's old town. It has not yet been finally decided whether cars will also be able to drive over the new bridge.^[10]

In addition to the changes in the city centre, the Wallgraben – Dürrlewang route was put into operation with the 2016 network, and the Hauptbahnhof – Budapester Platz – Milchhof and Hallschlag – Bottroper Straße – Wagrainäcker sections of the U12 were put into operation with the 2018 network. Since 2017, the U12 light rail line has been running with double trains from Dürrlewang in the south via the city centre, through the Europaviertel , the Nordbahnhofviertel and Hallschlag down into the Neckar valley to Remseck in the north.^[11] Since then, the U14 no longer runs to Remseck, but only to Mühlhausen. The extension of the southern branch between Dürrlewang and Wallgraben went into operation on 13 May 2016. Initially, the U14 was to end at the Max-Eyth-See,^[12] but in March 2013 it was decided that it would run to Mühlhausen. For this purpose, a reversing track with a 40-metre-long platform was set up at this stop.^[13]

The sections of track added in 2016 and 2017 are primarily intended to provide better connections to the previously neglected district of Hallschlag, the Europaviertel currently under construction , the planned Rosensteinquartier and other new development areas planned as part of Stuttgart 21 on the existing tracks. The entire route is designed for 80-meter trains.

The first new section of the U12 was the 2040 meter long construction section between the Löwentor and Hallschlag stops, which was completed in September 2013. The route runs mostly in the middle of Löwentorstrasse between the two lanes on the green strip that has been kept clear for many years. Around 200 trees were felled on the former green strip to build the route. Eight larger trees were saved and replanted. Young trees are being replanted on both sides of the lanes, creating an avenue on large parts of Löwentorstrasse. Since then, there has only been one lane for private motorized traffic in each direction, and a cycle lane has been added. The three intersections Hunklinge, Züricher Strasse and Auf der Steig have been converted into roundabouts. During the construction of the Pragsattel tunnel, the tracks in the area of ​​the Löwentor intersection were laid as a preliminary work. The construction section included the four stops Löwentor, Züricher Strasse, Riethmüllerhaus and Hallschlag. They are equipped with 80-metre-long elevated platforms to enable later operation in double traction.

The light rail tunnel between the Hauptbahnhof (Arnulf-Klett-Platz) and Stadtbibliothek stops was rebuilt in a different location due to Stuttgart 21 (see new construction of the Heilbronner Straße tunnel). The U12 uses this new tunnel until shortly before the Stadtbibliothek stop and then branches off underground towards Budapester Platz . In the Europaviertel, the U12 route runs largely in a tunnel and, after passing under the Stadtbibliothek at Mailänder Platz, reaches the surface on the east side via a ramp, after which, after crossing Lissabonner Straße,^[14] the Budapester Platz stop (planned name Wolframstraße ) follows on a bridge. The 145-meter-long bridge then leads over the Wolframstraße/Nordbahnhofstraße intersection and along the side of Nordbahnhofstraße to the Friedhofstraße/Nordbahnhofstraße intersection. There it connects to the ground-level light rail tracks in the middle of Nordbahnhofstraße.^[15] In the 1990s, the U12 was scheduled to be commissioned by 2001, with construction starting in 1998.^[16]

According to the plan, the U15 line was also supposed to run along this route. After the SSB initiated a change to the planning approval decision in early 2008, the route runs above ground in a lateral position on Nordbahnhofstrasse and swings into the middle position at about the level of Friedhofstrasse and connects to the Milchhof stop. Originally, an underpass at the Nordbahnhof-/Rosensteinstrasse intersection was planned. 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 route branch on Friedhofstrasse. With the commissioning of the new section, the section on Friedhofstrasse and the existing Pragfriedhof stop , which were already intended as temporary measures when they were set up, were closed on December 2, 2017.^[17] In order to continue to offer nostalgic tram service with the former meter-gauge trams, the new line through the Europaviertel is designed as a three-rail track.

In another section, the U12 connects to the U14 line in the north. From the Hallschlag stop, the route runs in a trough south of Löwentorstraße and reaches the new Bottroper Straße stop. This is followed by a 500-meter-long tunnel that takes it into the Neckar valley and under the Schusterbahn. Austraße is then crossed with a newly built bridge and joins the U14 line at the Neckar bridge. This section of the route is around 1.1 km long. The next stop is Wagrainäcker. The gap was closed when the timetable changed on December 9, 2017, with small opening celebrations and a parallel journey.

Due to the construction of the Rosenstein Tunnel for the B10, the Wilhelma stop was rebuilt in 2016 in a different location, directly in front of the main entrance to the Wilhelma.^[18] Likewise, in May 2019, the Rosensteinpark stop was rebuilt in its final location between the two tunnel ramps on Pragstrasse. Due to the excavation pit for the southeastern tunnel portal, the route between Wilhelma and Mineralbäder or Mercedesstrasse ran on a single track over a temporary bridge and had to be relocated again after its completion. Since summer 2020, all routes in the Neckarstrasse, König-Karls-Brücke and Leuzebad areas have been back in double-track operation. After completion of the work on the New Rosenstein Bridge, only a few remaining works remain to be done.^[19] If the old Rosenstein Bridge, which will then be redundant, is demolished after the Stuttgart 21 station project has been commissioned - something about which there is still no clarity - further temporary structures for the U14 will be necessary during the demolition period.

The U6 was extended by just over three kilometers from the Fasanenhof Ost industrial estate to the Stuttgart trade fair and the airport. Just south of the former terminus Fasanenhof Schelmenwasen, the route crosses the A 8 after a short gradient of 8.5%^[20] with a bridge east of the Echterdinger Eis, which was later awarded first prize by the Austrian Steel Construction Association for infrastructure projects, the German Engineering Prize and the German Bridge Construction Prize . The route then runs along the B 27 with the stops Echterdingen Stadionstraße and Messe West. It then runs north of Flughafenstraße to the terminus Flughafen/Messe Ost near the existing S-Bahn station and the planned Stuttgart Airport long-distance train station.

On 14 November 2012, the state of Baden-Württemberg decided to fund the U6.^[21] The state therefore covered 20% of the eligible costs, and the federal government 60%.^[22] In the cost-benefit analysis, the U6 extension performed better than a trade fair connection with the U5 from Leinfelden. Therefore, the U6 extension was pursued, although both routes would be economical.^[23] In late summer 2013, the negotiators of the cities of Stuttgart and Leinfelden-Echterdingen , the state, the SSB and the district of Esslingen agreed on the financing.^[24] In September 2013, the regional assembly approved the agreements and thus approved the construction of the U5 (see U5 extension to Neuer Markt ), the U6 and the S2. Work to clear the construction site began in October 2017.^[25] The cost forecast was originally 70 million euros and was revised to 94 million euros in 2016 and 101.3 million euros in 2017.^[26][22][27] In 2019, the SSB reported a further cost increase of 25 to 30 percent.^[28]

The official commissioning took place on 12 December 2021 with the timetable change. The introductory journey took place on 11 December 2021.^[20] Afterwards, at around 4 p.m., the route was opened to the public.