On the Berne-Olten-Zurich railway line in Switzerland, the stretch between Gretzenbach and Wöschnau, which is the sole remaining twin-track section, forms a bottleneck. This situation is to be rectified by late 2020 by widening the line between Olten and Aarau to provide four tracks. This upgrade will increase the route’s availability and capacity as well as it will attain more flexibility. We reported on the 3.1 km long Eppenberg Tunnel in issue 8/2017 of tunnel. This article thus merely provides a brief summary of the overall project and concentrates on the findings obtained from the construction sections already completed as well as the current status.