The original S-Bahn has been so successful that Zurich voters approved construction of a second underground through route called the Zurich Cross City Link or Durchmesserlinie (German). This is a line under the Hauptbahnhof to Oerlikon Station. The project includes a four-track underground station at the Hauptbahnhof (Löwenstrasse Station), a long railway viaduct between Altstetten and the Hauptbahnhof, the Weinberg Tunnel (map) and extensive improvements (including addition of a track) to the Oerlikon Station. The Löwenstrasse Station and Weinberg tunnel were opened in 2014. Work is continuing on the viaduct and Oerlikon station improvements. The route currently serves S-Bahn trains but long distance trains will be added in the coming years.
All the major public transport projects currently underway or in the planning process are described on the ZVV Projects (German) page.