The original tunnel was opened during 1875; it is set to be closed to rail traffic during 2020-2021 following the commissioning of its replacement, after which it shall be converted and retained for service and rescue purposes in respect to its successor.

[5] However, during the 2010s, the original tunnel's loading gauge was deemed to be insufficient and that work would be necessary in order to realise ambitions for this corridor to provide a headroom of 4 metres (13 ft) throughout its length, allowing for the haulage of taller freight trains, resulting in a reduction in the numbers of lorries traversing the road network.

[7] Studies were conducted into proposed remedial work to expand the loading gauge, which found that such efforts would involve a six-year timeframe that would heavily impact existing traffic operations throughout; an alternative approach of constructing a larger second tunnel alongside the original gained favour instead.

[8] The Swiss Federal Office of Transport permitted the construction of a new Bözberg railway tunnel, to parallel the existing one, at a cost of SFr 350m.

[11] The excavated spoil was removed from the tunnel by rail, using three trains per day to transfer a daily average rate of 1,800 tonnes of material to Wildegg, where it was subsequently used in restoration work at the Oberegg limestone quarry.

[11] Progress within the tunnel was relatively steady, despite varying geology along its path; the rate of advance was sometimes as high as 40 meters per day, achieve by crews working in two shifts of eight hours each per workday.