According to German press reports, German Rail (DB) says the main reason for the increase in costs is that tenders for construction contracts have resulted in much higher prices than expected due to a general construction boom, while reissuing the tenders would only have delayed the project even further. DB also says that a study it commissioned into the project has confirmed that work will take longer than expected to complete.

The big question is who will cover the increased costs. The original financial agreement between the state of Baden-Württemberg, Stuttgart city council, the local transport association, Stuttgart Airport and DB was for €4.5bn. All of the partners except DB believe that DB is responsible for any cost overruns. Some politicians are calling for the project to be halted as they believe it is now uneconomic. DB’s supervisory board is scheduled to meet on December 13 to discuss the problem.

The project has been dogged by controversy. Violent protests against Stuttgart 21 erupted in 2010, but voters in a referendum in Baden-Württemberg in 2011 voted in favour of continuing.

Stuttgart 21 entails the construction of a new underground central station, a 9.5km tunnel from there to a new airport station to join a new 10km line to Wendlingen where it will connect with a new 60km high-speed line from to Ulm. A second tunnel will run under the south side of the city to Obertürkheim, where it will emerge to join the existing line to Esslingen. A new chord south of Vaihingen will allow trains arriving from the Singen direction to serve the airport station and run into the main station from the south.