FOR passengers arriving in Stockholm from the south, the city's island geography is plain to see as the train sweeps across the Årstaviken and Riddarfjärden, offering panoramic views across the water on the approach to Stockholm Central station. But this environment also creates challenges for railway operations. The line is the busiest double-track railway in Sweden, and every weekday infrastructure manager Trafikverket squeezes up to 24 trains per hour in each direction through the so-called "wasp's waist" between Stockholm South and Stockholm Central with commuter, regional, long-distance and freight services vying for paths.

Eight out of 10 train journeys in Sweden begin or end in Stockholm, and all services to and from southern Sweden, the most populous part of the country, must negotiate this section of line. Furthermore, the city's commuter rail network carries 250,000 passengers per day and ridership continues to rise as Stockholm's population grows. With freight and long-distance passenger traffic also on the increase, the wasp's waist is not just a headache for commuters but a weak link in the entire region's rail network.

Proposals for engineering solutions to this pinch point go back decades, including adding a third track and tunnels on various alignments. In the 1990s work started on the construction of a third track, but it was soon decided the project would not offer sufficient additional capacity and it was subsequently abandoned.

In 2000 the Swedish government asked Banverket (now Trafikverket) to study a permanent solution to the problem and in 2002 a feasibility study was completed on what would become the City Line, a 6km double-track tunnel beneath the city centre which would allow commuter services to be diverted underground and release much-needed capacity on the existing surface line.

The plan was formally adopted in 2005, but the following year a coalition government was elected which felt the City Line was too costly and too complex. A public inquiry was subsequently launched, but this concluded a tunnel was the most practical and economic option for solving Stockholm's rail capacity problems. In May 2007 the government finally authorised Banverket to proceed with the project, clearing the way for construction to start in January 2009.

citylineThe SKr 16.8bn (SUS 2.58bn) tunnel is being financed jointly by the Swedish government through Trafikverket (SKr 10bn), the city of Stockholm (SKr 1bn) the region of Stockholm SKr 3.8bn), and the regions of Mälardalen and Östergötland (SKr 2bn).

"It was not easy to get the project accepted, and it was discussed at length by politicians for a long time before the government decided to go ahead," explains City Line project director Mr Kjell-Åke Averstad. "There are a lot of technical challenges in this project. If you look at the layout of the city on the surface, you can see there is a lot of water and a lot of old buildings. Things are just as complicated below the surface, because the tunnel has to pass between a lot of existing tunnels. It's very difficult to drive the line through all of this while providing good connections to the metro and bus routes from the new stations."

Construction of the running tunnels and service tunnels was tendered as seven sectional contracts, which vary in their value (SKr 250m to SKr 1.5bn), complexity, and tunnelling method. "We wanted to offer the market a variety of contracts, firstly to ensure competition, but also because we have some specialised works which are new in Sweden, where we needed to bring in experience from abroad," says Averstad. "There is a lot of flexibility in the contracts. For example, one contractor can take on tunnelling work in the adjacent section if he reaches the border first. This means that if one contractor experiences delays it won't necessarily hit the project schedule."

The northern portal section at Tomteboda and the southern portal near Stockholm South are lined concrete tunnels, while four of the five intermediate sections are blasted straight through the bedrock.

South of Stockholm Central the tunnel encounters its biggest obstacle, the Riddarfjärden, and this requires the construction of a 300m-long immersed precast concrete box supported by steel-reinforced piles to carry the line under the channel between Södermälarstrand and Riddarholmen. This was the only one of the seven tunnelling contracts to be let on a design-and-build basis, and the tunnel is being constructed by a consortium of Züblin, Germany, and Pihl & Son, Denmark, which were involved in building a similar structure on the Øresund Link, while the design work was carried out by engineers from Cowi and Züblin.

The three tunnel sections, which are each 100m long, 21m wide and 10m tall were cast in two stages in large steel cradles. The first stage was carried out at Södertälje on Lake Mälaren, where the lower part of each section was cast. All three sections were then towed by a tugboat to the construction site, where the remaining concrete was added. The first section will be lowered into position in the spring, and the remaining two will be sunk during the summer.

Beyond the southern portal a 1.4km flyover is being constructed between Årsta and Älvsjö to carry the City Line over the northbound track of the existing line from Stockholm Central.

New stations

The project involves the construction of two new stations, the first mainline stations in Sweden to be equipped with platform screen doors. At the northern end of the new line, Odenplan will replace the existing Karlberg station with a single island platform and two tracks, although it has been designed to allow expansion to four tracks in future when required.

Situated just east of Stockholm Central, Stockholm City will be located directly below T-Centralen metro station and will have two island platforms from the outset. The station's two underground concourses will have direct escalator links to T-Centralen, the hub of the metro network, significantly improving links between metro and commuter rail lines. Both stations will have 225m-long platforms to accommodate 107m-long six-car X60 emus operating in multiple.

While Trafikverket will own and operate the railway, SL will be responsible for the two new stations.

By the start of January 90% of blasting had been completed in the tunnels and tendering was underway for the contracts to fit out the stations. As IRJ went to press, Trafikverket was also preparing to launch a further major tender for the electrical and mechanical (E&M) contract, encompassing track, signalling, telecommunications, electrification, and traction power supply. One challenge facing the winner of the E&M contract is that civil works on the tunnel will not be complete when the installation of railway systems begins. "Bidders will have to show us how they will deal with this issue," says Averstad. "Clearly access will be limited, although the service tunnel will be finished so it will be possible to move small machines into the running tunnel."

Building a new railway through such a densely populated area requires the support of the community, and Trafikverket has carefully structured its communications strategy to keep rail users, local residents, and property owners up-to-date on the project and potential disruption during construction. The City Line team also engages directly with the public through social media, with a Facebook page and dedicated YouTube channel providing regular project updates. "Our common goal is to build the City Line while minimising disruption to the city and preserving the environment," says Averstad. "The public accepts the project and understands why the work is being done. 95% of people in this city support the project, and that is a real achievement. It's very important to have high-quality dialogue with the people who are affected by the work, explain to them why we are doing this, and give them a chance to ask questions."

Following a six-month period of testing and driver familiarisation, the line will open in 2017 and will initially operate using the standard Swedish ATC system. However, lines in the Stockholm area are expected to migrate to ETCS towards the end of the decade, and Averstad expects the City Line to make the transition in 2019-20. At least 16 trains per direction per hour will operate on the City Line at peak times and the line is designed for 24 trains per direction per hour, effectively doubling capacity through the city centre. "SL wants to operate more services, the regional authorities want more regional trains, demand for long-distance and freight paths is rising," says Averstad. "It's clear none of this can be accommodated without the City Line. The tunnel also needs to be in operation before additional capacity is needed for high-speed trains on the surface line."

The City Line has drawn heavily on the experience of other recent tunnelling projects in the region, notably the Øresund link, the Hallandsås tunnel and Malmö Citytunnel, and the expertise gained in Stockholm is already benefiting similar projects. Engineers planning Gothenburg's Västlänken tunnel visit the City Line on a regular basis, and Averstad says the project has had a lot of interest from overseas.

While the City Line will leave a welcome skills legacy for tunnel construction in Sweden, its biggest impact will be felt in rail operations not just locally but across southern Sweden, eliminating a weak link in the network and meeting the capacity demands of a growing railway.