It's between the underground tracks of Bahnhof Löwenstrasse and the above ground: [1]

There are a lot of little neat things. For example the elevators go sideways [2] because the platform in the underground Bahnhof is wider (this is due to safety regulations etc.) [3]

BTW, what is marked as Stadttunnel in [1] is the new bike tunnel [4] which has actually been there for many years as it was supposed to be a highway tunnel but was never opened (built many, many years ago).

[1] https://img.nzz.ch/2014/05/30/1.18312867.1401474596.jpg

[2] https://www.standseilbahnen.ch/images/8000/8000.06-zuerich-h...

[3] https://implenia.com/fileadmin/implenia.com/referenzen/durch...

[4] https://www.stadt-zuerich.ch/content/dam/web/de/mobilitaet/v...