In article you write:
I know that Seattle converted at least one tunnel, which was originally
to be for a light rail line, into a dedicated bus lane.

Nope. They put in rails when the tunnel was built in anticipation of
rail service, but only ran buses for about a decade. Then when they
were ready to build the long delayed light rail line, they found that
the rails in the tunnel were no good and spent a lot of money to tear
them out and put the current rails in. If you take the light rail
from SEA airport into Seattle, it ends up in the tunnel. If you get
off at the Westlake station, you can go upstairs and take the Monorail
("it's the transportation of the future and always will be!") to the
Space Needle.