Station Landstrasse, served by lines U3 and U4, is the large transfer station. Take the U4 line to the Hütteldorf direction. From the Landstraße stop, pass over the Stadtpark stop and exit at Karlsplatz stop. The Vienna U-Bahn operates daily between 5 am and 12:30 am.
By User:My Friend - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=5020437