The Koralm Railway Line represents an essential section of the Pontebbana corridor between Graz and Klagenfurt. It creates new capacities and significantly improved conditions for the freight transportation. Furthermore, the travel time of the passenger traffic will be considerably reduced. The heart of the Koralm Railway Line is the Koralm tunnel with a length of approximately 32.8 km. During excavation of the partly extremely demanding neogene sections of the investigation tunnels for the Koralm Tunnel, valuable experiences can be gained concerning the drainage and the closely related excavation behaviour, in addition to the geological-geotechnical insights. In the paper, the predicted mountain water conditions of the tunnel sections within neogene sediments as well as experiences with the implemented dewatering measures are described. These experiences strongly influence the detailed design of the future main tunnel structure.