You may be able to walk in the State St entrance as far as the intersection that goes to the 360 Garage, but will probably get lots of funny looks. There's a loading dock there for the apartments, I walked through there to help a moving truck get in so it's physically possible. Past that intersection though is a guarded gate. The Elm St entrance has the guard gate right at the start so you'd only be able to look in from there. The tunnel mostly serves various parking garages / loading docks.