SEVERAL years later than planned, the new underground main station that forms the centrepiece of the Stuttgart 21 project is due to open in December 2025. It has involved the construction of a complex network of new lines, mostly underground, to connect the new station to lines radiating from Stuttgart. However, questions remain unanswered as to whether the new station will be able to provide sufficient capacity to handle the 46 trains an hour planned for 2030 under the Deutschland Takt national regular interval timetable.

Stuttgart 21 has been highly controversial in Germany since its inception, with opponents citing everything from the environmental damage caused to the expense of mostly underground construction. Its proponents, principally the City of Stuttgart and German Rail (DB), have pointed to the improved services made possible by the new through station in place of the historic terminus, as well as the potential for massive urban renewal on sites currently occupied by the old station and DB maintenance facilities, all of which will be demolished if the current masterplan is followed.

The project, as originally conceived, was to replace the terminus with over 16 platform tracks with a new through station with only eight. Advances in both train acceleration and braking, plus new digital signalling - Stuttgart is Germany’s first “Digital Node” for the rollout of ETCS - would require fewer tracks, especially as trains would no longer have to reverse in the terminus.

But since Stuttgart 21 was first developed in the 1990s many things have changed, not least the planned introduction of Deutschland Takt with increased service frequencies that are expected to grow passenger numbers. The planned increase in services makes the new station, with only eight tracks, seem impractical. While trains will no longer reverse, crews will still change (adding time while train brakes and other systems are checked) and large numbers of passengers will board and alight, all leading to station dwell times of five minutes or more, as seen daily at other key German rail hubs such as Hannover or Cologne.

Local politicians and passenger representatives have raised their concerns that the design of the station will reduce overall network capacity as double platform occupancy will not be permitted.

The current plans for Deutschland Takt call for 46 trains an hour through the new station, with an extra 10 per hour at peak periods. This equates to a minimum of 5.75 trains an hour per platform face, with dwell times of between 2 and 5 minutes for long-distance services, which in practice are unlikely to be achievable. Any major rolling stock or infrastructure failure would potentially lead to congestion. In many stations, including the existing terminus at Stuttgart, the solution would be to allow two trains to occupy some or all platforms simultaneously, especially shorter regional trains. However, it appears the new station will be unable to do this as its designers have built the platform tracks on what, for a major station, is a very steep gradient of 1.51%. Initial concerns were raised in 1992 and have been publicly discussed in the German trade press since railway engineering consultant, Mr Sven Andersen, began writing about the issue.

Local politicians and passenger representatives have also raised their concerns that the design of the station will reduce overall network capacity as double platform occupancy will not be permitted. This has also led to calls for a smaller version of the existing terminus to be retained to accommodate some of the trains that would still terminate in Stuttgart.

Local MP, Mr Matthias Gastel, has been a long-term critic of the Stuttgart 21 project, as well as a leading advocate of Deutschland Takt. Since May 2022 he has represented the Green Party on the supervisory board of infrastructure manager DB Network. Gastel has obtained details of the planned operation of the new station, which appears to confirm the need for double platform occupancy, but he says that it is unclear “whether this can be approved at all in view of the steep track gradient.” A spokesman for Gastel told IRJ that “there is little new information regarding the capacity of the station. The commissioning timetable is slowly taking shape, but has not yet been finalised.”

TSI non-compliance

Andersen has studied the plans for the new station in detail for many years and has highlighted its key design weakness. The 1.51% gradient on the underground platform tracks represents a six-fold increase on the maximum allowed by regulations and practice in Germany, where it must not exceed 0.25%, and elsewhere.

This aspect of the station design was highlighted over 30 years ago when the plans were first unveiled, and when the project was revived early this century there was no redesign to provide level tracks. This was because of the need to avoid existing tunnels, and in particular for the new tracks to pass over the concrete box for S-Bahn services that runs at a 90° angle under the western end of the new platforms. At the eastern end, the new underground tracks must pass beneath other tunnels also at an angle of around 90°.

The non-compliant gradient first proposed in the 1990s does not meet subsequent standards set by European Technical Specifications for Interoperability (TSI). While TSI Infrastructure has been revised several times since it was first issued in 2002, the current version says that gradients of tracks through passenger platforms on new lines shall not be more than 0.25% where vehicles are regularly attached or detached. As multiple-units are routinely attached or detached at the current Stuttgart station, it would appear this stipulation would apply. If so, even more operational flexibility would be removed by preventing operators from doing this in the new station.

Andersen points out that the historic approach to maximum gradients in platforms was designed to prevent trains running away, injuring passengers as they are boarding or alighting, or potentially colliding with other trains. Experience since the 1950s in Germany has shown that even shallow gradients can cause real problems with modern roller bearing axles having less rolling resistance leading to potential runaways on slopes greater than 0.16%. Cologne main station was built on a 0.3% gradient to enable tracks to reach the adjacent bridge over the River Rhine, and 17 incidents of trains running away were recorded there between 2010 and 2014 alone.

For Andersen, good practice has been demonstrated by the new platforms beneath Zürich main station in Switzerland. Despite a maximum approach gradient of 3.7% from the surface, the tracks in the new Löwenstrasse underground station are level (see graphic p18). Andersen told IRJ he has written to DB Network about his concerns several times, personally delivering a letter to DB board member for infrastructure, Mr Berthold Huber, at InnoTrans in Berlin in September last year. DB has yet to reply. The railway told IRJ that it would not make a statement on the issue.

Safety concerns

The independent pro-public transport pressure group Verkehrsclub Deutschland (VCD) has been attempting to get clarity on the impact of the steep gradient on operation and timetables. The local VCD Baden-Württemberg branch has asked the Federal Railway Authority (EBA) to prohibit double platform occupancy in the new station. While the authority has declined to do so, it has neither refuted the fact that the gradient of 1.51% is six times the legal norm, nor that this will lead to a ban on double occupancy, instead saying that this is a matter for the final commissioning permit process. The EBA has also confirmed that the necessary “proof of equal safety,” confirming that the design met the prevailing standard requirements, was not issued when the project was first approved “because this proof was not requested.”

VCD Baden-Württemberg chairman, Mr Matthias Lieb, told IRJ that the “the question of the safety of the new Stuttgart 21 underground station with regard to the gradient has actually not been clarified.” He added that, based upon the wide variation between the 1.51% gradient and the permitted maximum of 0.25%, that if the approval process prior to opening “were to agree to using a proof of equal safety, the station would not receive a commissioning permit.”

“The question of the safety of the new Stuttgart 21 underground station with regard to the gradient has actually not been clarified.”

Mr Matthias Lieb, VCD Baden-Württemberg chairman

However, Lieb added that the Federal Ministry for Digital Affairs and Transport (BMDV) has told VCD that there were no official documents to prove equal safety with regard to track gradient, and that the “proof of equal safety” regulation did not describe the “recognised rules of technology,” but was an administrative standard. Therefore, according to the ministry, “proof of equal safety” is not necessary and it is sufficient to “prove safety.”

“From our point of view, it would be important to have clarity on this question at an early stage but, unfortunately, DB and EBA do not want to provide clarity on this at the moment,” Lieb says. “In our opinion, the timetable is currently planned with double occupancy, but for safety reasons such double occupancy should be prohibited.”

He adds that the refusal to provide clarity on what will be the actual capacity of the new station helps those who do not want to consider alternative solutions, including a new additional terminus. Lieb singles out the City of Stuttgart, which he says is refusing to hold “objective debate about an alternative station that is compatible with urban development.”

New terminus

A report undertaken for the Baden-Württemberg state government by Obermeyer Planen + Beraten and published in mid-2021 found that building an additional underground terminus for regional and S-Bahn services alongside the new through station at a cost of €785m would be both technically feasible and compatible with redevelopment of railway land.

The report was welcomed by Baden-Württemberg transport minister, Mr Winfried Hermann, who described the proposed additional station as “an essential requirement for the expansion of S-Bahn and regional train services,” while also providing an alternative if operations were disrupted in the new through station.

The platform tracks for the new underground station have a 15.14% gradient, which represents a six-fold increase on the maximum allowed by regulations and practice in Germany, where it must not exceed 2.5%. Image: Plan B, Stuttgart

He remains convinced that an additional station is needed, or will be from 2030. A public commitment to plan such a station has been made in the coalition deal between Hermann’s Green Party and the conservative CDU that formed the current Baden-Württemburg state government in 2021, which means that this issue will not go away quietly. The likely refusal of federal regulatory bodies to sanction double platform occupancy in the new through station due to the excessive gradient will probably hasten the decision to add new capacity as part of the redevelopment of the current surface station.

Hermann has said he supports a “supplementary” underground terminal station for regional trains, which would be relatively easy to build when clearing the existing site, although connecting it to the network would imply either the retention of older infrastructure that DB Network is planning to close or building even more new tunnels.

While other project sponsors such as DB, the City of Stuttgart and transport authority Verband Region Stuttgart all currently dismiss the objections from the state government and others, saying that the new station has enough capacity to deal with existing and future traffic, this has yet to be proven in practice. It appears likely to many observers that simple operational problems such as higher passenger numbers, train defects, late running connecting trains or simply staff being delayed will lead to station dwell times being significantly longer than planned. Without double platform occupancy or extra platforms, following trains will have to wait in the tunnels approaching the new station.

The outcome of these discussions to date suggests that the final approval for opening the new station could be delayed due to administrative or legal arguments over the safety of train operations due to the steep gradient. The more likely alternative scenario is that the station will open but with operating restrictions in place that ban double platform occupancy and the splitting and joining of trains, and potentially increasing block spacing requirements under ETCS that reduce capacity but avoid the risk of collisions.

Whether the opening is delayed or capacity is reduced, it still remains to be seen whether the full Deutschland Takt timetable can be provided. However, less than 34 months from the planned opening day it appears very likely that the additional station proposed by the state government will be needed and will eventually be built.