AROUND 400,000 passengers board, alight from or change trains in Zurich every day, and by 2020 this figure is expected to reach half a million, which means completion of the city's second cross-city line - the Durchmesserlinie (DML) - cannot come soon enough. The line, which includes a second underground through station beneath Zurich main station, will not only relieve congestion in the city but will also play a key role in the enhancement of Switzerland's east-west axis by cutting journey times for services on the Geneva - Lausanne - Bern - Zurich - St Gallen corridor.

Starting in the west, the 9.6km line consists of the following main components:

two single-track viaducts between Altstetten and Zurich main station, which include the 394m Kohlendreieck bridge and the 1156m Letzigraben bridge

an underground four-track through station with two island platforms at Löwenstrasse which will be about 1km long

the 4.5km double-track Weinberg tunnel between Zurich main station and the south of Oerlikon, with a parallel escape and rescue tunnel, and

• widening of the cutting between the tunnel portal and Oerlikon station by up to 18m to accommodate two additional tracks and expansion of the station area.

The total budget for the DML is SFr 2bn ($US 2.1bn) at April 2005 prices. The most costly elements of the project are Löwenstrasse station and the Weinberg tunnel which will cost a total of around SFr 1.15bn. Upgrading works at Oerlikon will amount to about SFr 450m, with the City of Zurich investing a further SFr 80m. The two new bridges on the section between Altstetten and Löwenstrasse will cost about SFr 290m.

The main structural works started in 2007 and are scheduled to continue until 2014 for the Weinberg tunnel and Löwenstrasse station, 2015 for the bridges, with the works at Oerlikon due for completion in 2016.

Construction of the Weinberg tunnel and Löwenstrasse station was tendered as one main contract for the basic infrastructure and two secondary contracts for the overhead contact wire and communications systems, while the works in Oerlikon and Altstetten are being carried out by Swiss Federal Railways (SBB).

In September 2011, SBB awarded the contract for the main railway engineering works to the Arge Bahntechnik Durchmesserlinie consortium of RhombergBau, (lead contractor), ImpleniaBau, and Cablex. In July, following nine months of preparation, installation work began, starting with the Weinberg tunnel.

The DML project has presented a number of constructional challenges. Building two new island platforms and four tracks for Löwenstrasse station 16m below platforms 4 to 9 of Zurich main station, which dates from 1847, under live operating conditions required some complex construction techniques, such as diaphragm walls, jetting and cut-and-cover. The new underground station will be accessed via 57 escalators and 10 lifts.

A 35m-deep vertical shaft had to be sunk in Oerlikon before construction of Weinberg tunnel could begin. A tunnel boring machine (TBM) was assembled in the shaft and set up to work towards Zurich main station. The machine drove first through molasse rocks below some very sensitive structures, such as University Hospital, ETH Zurich and the studios of Swiss National Radio DRS. Excavated material was moved by conveyor belt to Oerlikon station, from where it was transported by rail to a landfill site. To pass beneath the River Limmat, just before reaching Zurich main station, the machine had to be modified to drive through groundwater- saturated unconsolidated sediments. The breakthrough at Zurich main station took place in November 2010 and the tunnel boring machine was subsequently dismantled.

The parallel tunnel for light vehicles was broken through and connected to the main tunnel by cross passages every 500m. This provides a means of escape and rescue for passengers and staff.

South of Oerlikon, the existing line is being widened to accommodate two additional tracks (pictured). New retaining walls up to 28m high on both sides of the track are clad with sound-insulating panels to protect neighbouring properties from train noise.

A rescue and equipment building will be built at the tunnel portal to serve Weinberg tunnel and the existing Wipkinger tunnel. It forms part of the tunnel escape and rescue concept and contains railway technical systems and rescue equipment.

Oerlikon station is being modernised and widened to provide space for the two additional tracks. The existing platforms and pedestrian subways are also being widened and the station's eastern entrance expanded.

To avoid adverse effects on rail operations, the infrastructure works (apart from tracklaying) are logistically serviced from the vertical shaft in Oerlikon. All materials and installation plant are lowered to tunnel level by a portal crane, while staff access is by lift.

Weinberg tunnel has several safety features. A fire extinguishing water supply with a 200mm-diameter main pipe will be installed. Take-off points will be provided every 250m. Installation of low-voltage, fibre-optic, safety and security system cables in the pipe blocks of the tunnel side walkways started in August.

In the event of an emergency stop by a train in the tunnel, passengers will alight and move along an illuminated escape walkway at the side of the track to the nearest cross gallery to reach the escape and rescue tunnel. The escape and rescue tunnel also provides access for rescue teams and is designed for use by ambulances or light vehicles.

A positive pressure system in the escape and rescue tunnel will ensure that smoke cannot penetrate the tunnel in the event of a fire. Two airlocks, each with two airlock doors, are designed to withstand a pressure of 50Pa. Track crossings are installed at portals and cross passages to allow passengers to cross the tracks to reach the escape and rescue tunnel.

The overhead electrification will be installed next summer. To ensure the tunnel profile is kept as small as possible, SBB has opted for overhead conductor rails instead of conventional contact wires.

All systems for operating Weinberg tunnel, such as ventilation, power supply, and door contacts, will be monitored and controlled from Zurich main station signalbox, but from 2016, control will be transferred to the eastern control centre at Zurich Airport station.

Preparatory work for tracklaying began in July. The DML will be equipped with slab track with rails laid on monobloc sleepers concreted into the track bed. Low-vibration-track (LVT) sleepers will be used to minimise the ground transmission of train vibrations. Calculations during the design phase showed that additional measures would be necessary to protect the surrounding residential areas adequately against vibrations, so SBB is installing a 550m section of mass-spring system (MSS) in the area of the south portal, while 515m lengths are planned at the north portal and near the radio studios. This is followed by pouring a 21cm layer of concrete.

When trains pass over the spring system, it compresses by up to 15mm and prevents the transmission of vibrations. Well-designed transition sections ensure that passengers are unaware of the train dropping in level. A total of 16 switches will be installed and concreted into the slab track, 12 of which will be situated in Löwenstrasse station.

A very tight curve in Weinberg tunnel has to be constructed on a very high crossfall because of the 150mm cant on the rails. This means that the flowing concrete has to be very stiff to ensure the superelevation stays within the required tolerances. On the other hand, the concrete must have good flowing properties to grout under the sleepers and keep air bubble formation and voids to a minimum.

As these two requirements conflict, solutions had to be found. The strict requirements for accuracy and surface quality are achieved using a saw tooth technique, already employed on other Swiss projects. This creates "grooves" for later track installation and placing concrete under and around the sleepers. Flowing concrete can be used for this task to ensure the highest precision for the crossfall. The other solution involves using fibre-reinforced concrete on the approaches to the portals for greater strength and weather resistance.

Other challenges in the project include coordination of the logistics for the supply of materials to workers involved in the installation works; coordination of bringing the works into use; testing of individual components, technical equipment rooms and the tunnel to enable SBB to bring the whole project into operation, including test running, by January 2014.

The first scheduled trains are due to start running through Löwenstrasse station and Weinberg tunnel to Oerlikon in June 2014, while the two viaducts in Altstetten will open to traffic in December 2015, bringing much-needed relief to Zurich's congested rail network.