In London, work is well under way on the $7.4-billion initiative to improve Thameslink, the city’s only north-south surface rail line. The ambitious project, which includes new railway infrastructure and renovated stations, will double the line’s capacity by 2018.

Remodeling the old Blackfriars Railway Bridge, which carries Thameslink over the River Thames, is the central element of the project’s first phase, due for substantial completion next year. Current work includes replacing the Blackfriars terminus and building a new station at the south abutment.

The 125-year-old, wrought-iron bridge is being widened with new steel arches and is getting a new steel roof. Moreover, to accommodate 50% longer trains, the bridge’s station platforms are being doubled in length from the north river bank to cross the full 300-meter-long bridge.

A few kilometers up the line, Farringdon Station is being rebuilt. Further, a new viaduct is taking shape to unblock a bottleneck on the south bank outside of the London Bridge terminus, one of the city’s busiest transportation hubs.

The London Bridge terminus will be rebuilt in a second, five-year phase at an estimated cost of $1.4 billion. It is planned to open in 2018, more or less coinciding with the completion of the larger, east-west Crossrail railroad tunnel project, which is under construction. But for now, Blackfriars Bridge is taking center stage.

The roughly 300-m-long, five-span bridge was built with deck girders supported on posts rising from 5-m-tall arches. All the metalwork was wrought iron. Spanning typically 60 m between stone and brick piers, the arches are set 1.7 m apart.

To widen the bridge to 30 m from 23.5 m, four sets of new steelwork arches are being added. The new design replicates the old structural form but replaces iron posts and girders with steelwork.

One new set of arches already has been added to the east side, partially cantilevering from the old structure. The west side is being extended by three arches, supported by the foundations of the old, red piers of a freight-train bridge, demolished in the 1970s.

New steelwork is being fabricated to allow for the large geometrical variations found in the old structure. “We are talking about 10 to 15 millimeters,” says Tony Westlake, project director with the structural designer Tony Gee and Partners, Esher. “A large amount [of the bridge] was built manually. No two arches were exactly the same.”

To ensure a good fit, the contactor surveyed each of the roughly 30,000 original rivet locations and matched them with bolt holes, drilled into the replacement steelwork, says Chris Evans, project director with the London-based main contractor, Balfour Beatty Civil Engineering Ltd.

Balfour Beatty is remodeling the bridge and building the new Blackfriars stations under a fee-based, target-price contract, valued at about $620 million. Its subcontractor, Watson Steel Structure Ltd., Bolton, is supplying and erecting 6,800 tonnes of steelwork, with 5,500 tonnes for the deck and the rest going into the roof.

During construction, Balfour Beatty must keep one of the two railroad tracks open and also ensure the safety of river and railroad users, says Evans. “We had five 70-tonne crawler cranes working on the bridge immediately next to a live railway,” he adds.

With virtually no storage space, the contractor is using barges both as work platforms and to transport materials and equipment to and from a logistics center 75 minutes downstream. Barges will deliver a total of about 14,000 tonnes of materials to the site and remove 8,000 tonnes of debris.

Evans says his team successfully achieved, in 2009 and 2010, key milestones set for the quiet, end-of-December holiday periods, when trains were stopped for six-day periods, allowing unfettered access. In the first period, the contactor demolished an elevated connection between the station and bridge and slid in a 22-m-long replacement without a hitch, says Evans.

This past December, Balfour Beatty transferred the active railroad track from the west side of the bridge to the newly widened east side, despite the heaviest snowfalls in recent history, says Evans. “If we hadn’t done it, then we would have lost a couple of months at least,” he adds.

Before completing the contract next July, the contractor’s next key milestone is to allow for 50% longer trains on the crossing by this December. Because the new 12-car trains “will virtually span the whole river,” Evans says, his team must also ensure passenger access through the new south abutment station.

About two kilometers east of the south side of the bridge, the contractor Skanska U.K. Ltd., Maple Cross, is squeezing a new, 433-m-long, $164-million two-track viaduct between buildings toward London Bridge Station in the shadows of the ancient Southwark Cathedral and the emerging Shard, the country’s tallest high-rise.

This May, the contractor slid a 71-m-long bridge over the busy Borough High Street next to the station. The rest of the viaduct is a conventional steel-and-concrete structure in a difficult location, cramped between buildings and crossing the hectic tourist spot, Borough Market. 

“Every time you put a shovel in the ground, you found something you weren’t expecting,” says Skanska’s project manager Susan Fitzpatrick. Archaeological artifacts, including 16^th century Delft pottery, are dispersed along the route.

Foundations are now done, and the viaduct structures are taking shape. In the contract’s final stages, Skanska will focus on reinstating buildings and infrastructure affected by the construction work before handing over the project late next year. “We’ve done the exciting bit,” says Fitzpatrick.