Built on behalf of King Ludwig II in 1853, it is still the only tunnel on the Munich-Lindau double-track railway line. However, due to water penetration and severe frost, icicles were repeatedly formed on the inner walls and on the tracks during the winter months, hindering rail traffic. In addition, the cross-section of the once 123 metre long tube was so narrow that trains could only travel slowly on the two closely adjacent tracks - at the end of the day at a maximum speed of 30 km/h. The railways were not able to travel at all.

Massive conversion for more comfort

To ensure that double-decker trains and trains with tilting technology will also be able to pass through safely in future, Deutsche Bahn decided in 2016 to carry out a fundamental conversion and comprehensive renovation of the tunnel. The cross-section was increased to 70 square metres, a new reinforced concrete inner shell was installed and retaining walls were installed at the portals, which extend the existing tunnel at both ends by 16 and 20 metres respectively. In connection with the tunnel project, Deutsche Bahn also had 9.5 kilometres of track between Immenstadt and Oberstaufen renewed, a new bridge and several culverts built, four points installed in Oberstaufen station and the signalling technology adapted.

Blasting through the mountain

The construction work began in April 2016, when Max Bögl's tunnel construction specialists used 3.5 tonnes of explosives to extend the tunnel by around 1.50 metres around its circumference, so that the distance between the tracks could be increased from 3.50 to 4.00 metres. In the course of the blasting operation, over 15,000 tonnes of rock had to be excavated and removed. Then the new pipe was lined with a waterproof reinforced concrete inner shell with a vaulted base. In addition to the widening, Deutsche Bahn had the railway tunnel extended to a total length of around 160 metres. This tunnel extension in conjunction with new retaining walls now provides sufficient protection against rockfall instead of the old tunnel portals. The line was opened to rail traffic at the end of 2016. Since then, the trains have been travelling through the new tunnel at a speed of 80 km/h. The new tunnel has been opened for traffic at the end of 2016.