There are several big stations in Vienna. The two main ones are the Westbahnhof and the Suedbahnhof.
The both stations are connected by rail but it is a very long jurney however both station are located probably 3-4 km apart.
Now they built a new tunnel (Lienzer Tunnel) connecting the Westbahn (not the station) to the station called Meidling. This enables to cut the travel times from western direction to Suedbahnhof which serves almost all trains towards south, north and east from Vienna. The service toward west is still the domain of the Westbahnhof.
The new Hauptbahnhof will be between the station Meidling and Suedbahnhof (the later will be demolished).
You can find the very detailed map on
www.wien.at