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. The tunnel serves Downtown Seattle, running west under Pine Street from 9th Avenue to 3rd Avenue, and south under 3rd Avenue to South Jackson Street. 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 four stations 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. The 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 owned by King County Metro and shared with Sound Transit through a joint-operating agreement signed in 2002. The Downtown Seattle Transit Tunnel is one of two tunnels in the United States shared by buses and trains, the other being the Mount Washington Transit Tunnel in Pittsburgh, and is the only one in the United States with shared stations.