The Zimmerberg Base Tunnel is a railway tunnel under the Zimmerberg mountains in Switzerland. It is divided into two sections, of which section I is operational and section II only proposed. Zimmerberg I is about 10 km long and links Zurich with Thalwil and bypasses a section of the Lake Zürich left-bank railway line.
The proposed 21 billion Swiss francs variant of the Rail 2030 project includes the Zimmerberg Base Tunnel II.
Part II will bring the length to about 20 km and will link Zurich with Zug. The intermediate exit to Thalwil would still be used by passenger and freight trains going to Chur as well as by international trains to Austria.