The building of the construction railway on the south side was delayed by many appeals and the groundbreaking ceremony near Naters took place on 12 February 1907. The line of the long construction railway, with an average gradient of 30 per thousand (3%) corresponded almost exactly to the alignment of the permanent railway line as built except that the deviations around many side valleys and spurs were bypassed by the actual line. Even so, the construction line had 34 tunnels with a total length of , of which 13 tunnels were enlarged to become part of the permanent line.