Semmering Base Tunnel

The Semmering Base Tunnel is an under construction 27.3 km long railway tunnel underneath the Semmering Pass between Gloggnitz and Mürzzuschlag on the Southern Railway in Austria. Upon completion of the tunnel, it will replace the 41 km long Semmering Railway as the main route on this section. In addition to the reduced distance, the speed limit in the Semmering Base tunnel will increase to 230 km/h compared to speed limits of 60 to 80 km/h governing most of the Semmering Railway, offering journey time savings of up to 30 minutes.

In addition to reduced travel times, the tunnel increases ease of use for freight traffic. The gradients of the Semmering Railway require the use of two locomotives; the reduced gradient of the new link will enable the transit of freight traffic using just one locomotive. The Semmering Base Tunnel and the Koralm Railway in combination will enable freight transit along the entire southern line using just one locomotive.

Construction began on 25 April 2012 and the link is expected to enter operational service in 2030, delayed from the original estimate of 2026. The total project volume was originally projected at about €3.1 billion; in early 2025, costs were estimated to rise to €4.2 billion.

Previously to the commissioning of the Koralm Railway in December 2025, the fastest Vienna-Graz and Vienna-Klagenfurt journeys on the Railjet services were 2 hours 35 minutes and 3 hours 55 minutes long, respectively. The Koralm Railway enables Graz-Klagenfurt journeys of approximately 45 minutes, and (by rerouting via Graz rather than Leoben) reduces Vienna-Klagenfurt journey times to around 3 hours 25 minutes. The Semmering Base Tunnel is projected to enable time savings of an additional 45 minutes, which will enable journeys from Vienna to Graz and Klagenfurt of 1 hour 50 minutes and 2 hours 40 minutes, respectively.