The Lainz Tunnel (German: Lainzer Tunnel) is a 12.8-kilometre-long, double-track railway tunnel in Vienna in service since 9 December 2012. Part of a tunnel system that includes the 13.4 km Wienerwald Tunnel, it connects Austria's Western, Southern, Eastern and Donauländebahn railways and greatly simplifies international and national connectivity in Vienna.
Lainz Tunnel Wikipedia
The single-tube, two-track tunnel starts in the west of Vienna about 2 km west of Wien Hütteldorf railway station in an underground interconnection with Wienerwald Tunnel, part of the new high-speed alignment of the Western Railway, where it is also connected to the original above-ground Western Railway. Is turns southeast and passes under a stormwater retention basin and the forested area of Lainzer Tiergarten in Vienna's 13th district Hietzing. Close to its western terminus it splits in two, with one double-track connection emerging just before Wien Meidling railway station on the Southern Railways and the other further south connecting to the Donauländebahn railway and the intermodal terminal at Inzersdorf opened in 2016. Passenger trains run at up to 160 km/h and freight trains up to 120 km/h. It is 12.3 km long (15.4 including connecting ramps) and equipped with ETCS level 2 as well as the legacy PZB control system
Its chief role is in east-west transit as part of the Magistrale for Europe. Previously, international passenger trains either terminated at Wien Westbahnhof station and passengers travelling eastbound had to make their way to Wien Südbahnhof station, or had to be reversed in Westbahnhof and travelled along a single-track connection through residential areas. In 2014, the new Wien Hauptbahnhof through station replaced this former terminus, and in 2015 most long-distance services were concentrated at Hauptbahnhof, with trains originating from or bound for the Western railway travelling through the high-speed Wienerwald Tunnel and continuing on through the Lainz Tunnel towards Hauptbahnhof and to destinations beyond. The tunnel is also heavily used by freight trains bound for Vienna's central classification yard by way of a branch turning near the eastern end.