Installation of the second and final railway line for Auckland’s City Rail Link has been completed this week — marking a key milestone in the $5.5b project.

The newly laid 3.4 km of track will carry northbound trains underground from Maungawhau Station at Eden Terrace and connect with the two new central city ones, Karanga-a-Hape and Te Waihorotiu, and downtown’s Waitematā Station (Britomart).

The southbound tracks were installed last year.

City Rail Link chief executive Dr Sean Sweeney said there was plenty to celebrate for both project teams and Aucklanders.

“Completing the two tracks brings us one big step closer to the next phase of the project — testing trains inside the tunnels mid-year, and from there handing CRL over next year to Auckland Transport and KiwiRail, who will operate and maintain it.”

The track was installed on one of the steepest railway sections in the country, dropping 70m from Maungawhau to Waitematā, which sits below sea level. At its deepest point, the City Rail Link runs 42m underneath the Central Motorway junction.

Link Alliance Project Director Francois Dudouit said that conditions under Auckland were challenging with tight curves, a steep gradient, and no room for heavy machinery to pass.

“We became New Zealand’s first construction organisation to gain a Rail Operators and Rail Access Provider licence under the Railways Act – an innovation that gave us the flexibility we needed to plan the job and get it done to the highest standard.”

Most of the tunnels were excavated by the project’s tunnel boring machine, named in honour of Māori rights activist Dame Whina Cooper, with other sections near Waitematā Station constructed by first digging trenches and then covering them over.

With the completion of the tracks, work on New Zealand’s largest ever transport infrastructure project shifts to train testing, landscaping and urban realm enhancements.

Lifts, escalators, security systems, CCTV, electricity, signalling and communications will be also be installed in the stations and tunnels.

When complete and fully operational, the City Rail Link will more than double Auckland’s rail network capacity by transforming Waitematā (Britomart) into a two-way through station.

It will carry up to 54,000 people an hour, the equivalent to three Harbour Bridges or 16 extra traffic lanes in the city during rush hour.