The two-lane tunnel will measure 2.6km in length and form a significant element in the Olten-Aarau railway (halfway between Basel and Zurich).

At present, the railway is being widened to four tracks by the Swiss government and the Swiss rail company SBB.

A 12.8m-diameter Herrenknecht Mixshield TBM will bore through the expected hard rock and wet soil conditions.

Work will also involve the construction of three emergency and escape tunnels, five shafts for technical areas, and niches for M&E installation.

The project is scheduled for completion by August 2019.