The Downtown Seattle Transit Tunnel (DSTT), also referred to as the Metro Bus Tunnel, is a 1.3-mile-long (2.1 km) pair of public transit tunnels in Seattle, Washington, United States. It was used only by buses from its opening in 1990 until 2005, and since 2009 it has been shared by buses and light rail. The double-track tunnel and its stations, except Convention Place, constitute parts of the Central Link light rail line, which continues north to the University of Washington station and south through the Rainier Valley to Seattle–Tacoma International Airport as part of Sound Transit's Link light rail network. Its five stations are also served by King County Metro and Sound Transit Express buses that leave the tunnel north via Interstate 5, south via the SODO Busway, or east via Interstate 90. The DSTT is the busiest section of the Link light rail network, with an average of over 10,000 weekday boardings. It is one of two tunnels in the United States shared by buses and trains, the other being the Mount Washington Transit Tunnel in Pittsburgh, Pennsylvania, and is the only one in the United States with shared stations. Light rail service through the tunnel began on July 18, 2009. A stub tunnel, branching from the main tunnel, was constructed under Pine Street to allow light rail trains to stop and reverse direction; it was later used as the first segment of a light rail extension to Capitol Hill and the University of Washington that opened in 2016. Plans call for the downtown transit tunnel to lose its bus service during the permanent closure of Convention Place station in 2019; from that point on, the tunnel will be used only by light rail trains.
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