Since the 1960s, there have been attempts to build a tramway tunnel in the centre of Karlsruhe to relieve the main shopping street of trams.
However, these plans always failed because of the cost of such a project.
As the volume of trams on the central corridor along Kaiserstraße continued to grow with the expansion of the tramway and the Karlsruhe Stadtbahn network in the 1980s and 1990s, further plans for a tunnel route were made to relieve the surface tram line.
After an initial rejection at a referendum in 1996, a modified plan was confirmed at a further referendum in 2002.
At the end of 2005 the so-called "combined solution" (Kombilösung) for the Stadtbahn tunnel completed the planning approval process.
In December 2008, the Karlsruhe regional council approved the details of the work.
 The Stadtbahn tunnel has been under construction since 2010.

The project consists of several parts:
The construction of a two-track rail tunnel in the east–west direction in Kriegsstraße between Durlacher Tor and Mühlburger Tor with a branch from Marktplatz to the south to the convention centre (Kongresszentrum Karlsruhe; about 3.5 km of tunnel with seven stops);
The construction of a two-track tram line in Kriegsstraße between Karlstor and Mendelssohnplatz, about 400 metres south of Kaiserstraße (length: 1.2 km)
The construction of a road tunnel under Kriegsstraße;
The dismantling of the surface tram tracks in Kaiserstraße between Europaplatz and Kronenplatz and in Karl-Friedrich-Straße/Ettlinger Straße between Marktplatz and Vierortbad.

It was estimated that the cost of the project to the city of Karlsruhe would be €496 million and that its "standardised" ratio of discounted benefits to cost (Standardisierte Bewertung) would be 1.186.
In June 2007, the Federal Republic of Germany, the state of Baden-Württemberg and the city Karlsruhe agreed on the allocation of costs.
In December 2008, the cost estimate was adjusted to €588 million to be funded by a federal grant of €311 million (60% of eligible costs), state funds of €103.6 million (20%) plus the city's share of €173 million (20% plus costs that are ineligible for federal and state funding).
Following the outcome of the tender, the final cost is estimated at approximately €630 million.

Financing and implementation is being handled by a company established for the purpose: Karlsruher Schieneninfrastruktur-Gesellschaft mbH. The project will relieve the inner city of tram and car traffic.
The urban environment will benefit as a result of the elimination of the tram tracks in the pedestrian zone and the rebuilding of Kriegsstraße.
The city will also benefit from the creation of additional capacity in the tram network.

The project is still very controversial, which was also reflected in the results of the two previous referendums.
The high cost and the disruption during construction are criticised in particular.
The necessity of the project continues to be questioned and alternative concepts have been proposed in the form of amended alignments.
A citizens' initiative against the combined solution was launched in the summer of 2009 to require a third referendum, but this failed in the administrative court.

The first sod was turned for the construction of Stadtbahn tunnel on 21 January 2010.
The first, preparatory work had already been carried out in 2009.
The Stadtbahn tunnel was originally expected to be completed by the end of 2016; this is now expected at the end of 2020.
The reconstruction of the Kriegsstraße started in 2015 and is expected to be completed at the end of 2021.
Since the 1960s, there have been attempts to build a tramway tunnel in the centre of Karlsruhe to relieve the main shopping street of trams. However, these plans always failed because of the cost of such a project. As the volume of trams on the central corridor along Kaiserstraße continued to grow with the expansion of the tramway and the Karlsruhe Stadtbahn network in the 1980s and 1990s, further plans for a tunnel route were made to relieve the surface tram line. After an initial rejection at a referendum in 1996, a modified plan was confirmed at a further referendum in 2002. At the end of 2005 the so-called "combined solution" (Kombilösung) for the Stadtbahn tunnel completed the planning approval process. In December 2008, the Karlsruhe regional council approved the details of the work.  The Stadtbahn tunnel has been under construction since 2010.
The project consists of several parts:
The construction of a two-track rail tunnel in the east–west direction in Kriegsstraße between Durlacher Tor and Mühlburger Tor with a branch from Marktplatz to the south to the convention centre (Kongresszentrum Karlsruhe; about 3.5 km of tunnel with seven stops);
The construction of a two-track tram line in Kriegsstraße between Karlstor and Mendelssohnplatz, about 400 metres south of Kaiserstraße (length: 1.2 km)
The construction of a road tunnel under Kriegsstraße;
The dismantling of the surface tram tracks in Kaiserstraße between Europaplatz and Kronenplatz and in Karl-Friedrich-Straße/Ettlinger Straße between Marktplatz and Vierortbad.
It was estimated that the cost of the project to the city of Karlsruhe would be €496 million and that its "standardised" ratio of discounted benefits to cost (Standardisierte Bewertung) would be 1.186. In June 2007, the Federal Republic of Germany, the state of Baden-Württemberg and the city Karlsruhe agreed on the allocation of costs. In December 2008, the cost estimate was adjusted to €588 million to be funded by a federal grant of €311 million (60% of eligible costs), state funds of €103.6 million (20%) plus the city's share of €173 million (20% plus costs that are ineligible for federal and state funding). Following the outcome of the tender, the final cost is estimated at approximately €630 million.
Financing and implementation is being handled by a company established for the purpose: Karlsruher Schieneninfrastruktur-Gesellschaft mbH. The project will relieve the inner city of tram and car traffic. The urban environment will benefit as a result of the elimination of the tram tracks in the pedestrian zone and the rebuilding of Kriegsstraße. The city will also benefit from the creation of additional capacity in the tram network.
The project is still very controversial, which was also reflected in the results of the two previous referendums. The high cost and the disruption during construction are criticised in particular. The necessity of the project continues to be questioned and alternative concepts have been proposed in the form of amended alignments. A citizens' initiative against the combined solution was launched in the summer of 2009 to require a third referendum, but this failed in the administrative court.
The first sod was turned for the construction of Stadtbahn tunnel on 21 January 2010. The first, preparatory work had already been carried out in 2009. The Stadtbahn tunnel was originally expected to be completed by the end of 2016; this is now expected at the end of 2020. The reconstruction of the Kriegsstraße started in 2015 and is expected to be completed at the end of 2021.
Trams in Karlsruhe