There are apparently underground passages connecting the hospitals on University Avenue and all the way to Queen's Park TTC station. I couldn't find a map or even many references to its existence. This Instagram post is the most I could find, but it's unclear if it's open to the public, or how to get there.
Anybody here has experience with it?
They are Not hard to find from Queen's Park TTC station. I've used them to go to doctor's appointments. IIRC you can get in at the South East side of the station.