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).
I have seen this comment on Reddit a few months ago, and some people were talking about it. They came to the conclusion that you cannot see the Sihl.