Switzerland's Albula Tunnel is an important engineering marvel from the beginning of the 20th century. The 5,864 metre-long tunnel at 1800 metre elevation in the Alps was built between 1898-1903. A total of 1,316 men built this tunnel with just picks and shovels in just 5 years. This engineering marvel part of the Rhaetian Railway network is a UNESCO World Heritage Site.

The 110 years-old tunnel shows its age in recent years with falling rocks and the difficulty with keeping up with the modern safety standards. There were two options: Repairing and upgrading the tunnel or constructing a parallel new tunnel. Due to the minor cost differential between two options, it has been decided to build a new tunnel. The new construction had the advantage of not disrupting the current rail traffic during the construction.

The Rhaetian Railway site a lot of interesting information about the history and the current construction, I am sure you will enjoy it.

https://www.rhb.ch/en/company/projects-dossiers/rebuilding-of-the-albula-tunnel