The Downtown Seattle Transit Tunnel (before Link Light Rail called the Metro Bus Tunnel) is a 1.3 mile tunnel beneath downtown Seattle. 18 King County Metro and one Sound Transit Express bus routes use the tunnel. These are all Express routes that run via freeways on each end of their tunnel trips. The Central Link Light Rail also does but only as far as Westlake Station. The tunnel has a total of 5 stations. All bus routes serve all five stations before terminating at the northern of southern end of the tunnel. The three middle ones are underground, Convention Place is just below grade and in the middle of a large layover area, and International District/Chinatown is in an open cut, a good portion of which has been covered over. The tunnel began construction in 1987 and opened in 1990 for buses. It had to be closed for exactly two years from September 24, 2005 to September 24, 2007 to retrofit it for light rail service that mostly consisted of lowering the roadway/trackbed for the trains and replacing the trolleybus wire with light rail wire. The tunnel opened with tracks for future light rail already in place but this was designed incorrectly for what was desired for Link. Light rail service finally began on July 18, 2009. For propulsion from the tunnels opening in 1990 to 2005 buses used dual-mode running via trolley wire propulsion through the tunnels before converting to a conventional engine for their runs to their far away express destinations (similar to the MBTA Silver Line today). Although all run entirely at the surface King County Metro still has 14 trolley routes serving 69 miles of two-way street wire. Today bus routes through the tunnel use New Flyer DE60LF hybrid buses all of which are articulated that run in Hush Mode while in the tunnel operating on electricity from the batteries on board only running the diesel engine sparingly to recharge the batteries.

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