When an industrial sealant was found to be leaking into the watercourse surrounding the Hallandsås Tunnel site on Sweden’s west coast 20 years ago, workers downed tools and everything stopped.

Construction began on Sweden’s longest rail tunnel in 1992. The tunnel had been a long-held aspiration of Swedish State Railways, which had first proposed a subterranean rail link through the Hallandsås Ridge in 1975. It was to replace the existing coastal railway, which dates back to 1885, and remove a known bottleneck, but 23 years on and it is yet to be completed.

The problems began five years into construction when Rhoca-Gil, the sealant used to keep out intruding groundwater, contaminated the surrounding watercourse, killing fish and poisoning livestock. The high levels of acrylamide, a known carcinogen, detected in the water sparked a major environmental investigation, closing the the project down indefinitely and becoming the subject of various court cases.

Years of remediation work followed and in 2003, the site was declared safe and work was able to restart.

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Now into its third decade, the £812 million project is nearing an end. Tunnelling was completed in 2013 in the western tunnel but even this process wasn’t without its issues. Freeze pipes had to be installed to chill an unstable 200-metre section of ground, named the Mölleback zone, to – 40 degrees celsius, creating spectacular underground ice caves, to allow tunnelling to be carried out safely.

By the end of the year, both east and west tunnels should be open for passengers. Tracklaying was completed at the start of this year and work is now underway installing and testing the electrical and signalling systems, and constructing three new stations – Förslöv, Barkåkra and Bastad.

Once open, the tunnels will boost capacity from four trains an hour to 24 trains an hour and increase the line speed from 80 km/h to 200 km/h. It will also allow freight carriers to double the weight of trains using the route.

It is a project more than 20 years in the making. Apart from one engineer, Kenneth Rosell, who has worked on the Hallandsås Tunnel from the very start, several generations have played a part in one of Sweden’s biggest infrastructure challenges.