The Downtown Seattle Transit Tunnel (DSTT), also referred to as the Metro Bus Tunnel, is a pair of public transit tunnels in Seattle, Washington, United States. The double-track tunnel and its four stations serve Link light rail trains on the 1 Line as it travels through Downtown Seattle. It runs west under Pine Street from 9th Avenue to 3rd Avenue, and south under 3rd Avenue to South Jackson Street. 1 Line trains continue north from the tunnel to Northgate station and south through the Rainier Valley past Seattle–Tacoma International Airport to Angle Lake station as part of Sound Transit's light rail network. The DSTT was used only by buses from its opening in 1990 until 2005, and shared by buses and light rail from 2009 until 2019. Bus routes from King County Metro and Sound Transit Express left the tunnel north via Interstate 5, south via the SODO Busway, or east via Interstate 90. It was owned by King County Metro and shared with Sound Transit through a joint-operating agreement signed in 2002; Sound Transit assumed full ownership in 2022. The Downtown Seattle Transit Tunnel was one of two tunnels in the United States shared by buses and trains, the other being the Mount Washington Transit Tunnel in Pittsburgh, and was the only one in the United States with shared stations. Though proposals for a rapid transit tunnel under 3rd Avenue were introduced in the 1910s and 1920s, planning for the modern bus and rail Metro Bus Tunnel only began in 1974. The King County Metro Council approved the bus tunnel proposal in November 1983, but construction did not begin until March 1987. The tunnel between Convention Place and Westlake stations was built using the cut-and-cover method, closing Pine Street for 19 months and disrupting access to the retail core. The segment from Westlake to the International District was bored with two tunnel-boring machines, heading north from Union Station and finishing within a month of each other. Tests of normal buses and the Breda dual-mode buses built specifically for tunnel routes began in March 1989; tunnel construction was declared complete in June 1990, at a cost of $469 million. Light rail tracks were installed in anticipation of future rapid transit service through the tunnel, but were later found to be poorly insulated and unusable for Link light rail. Soft openings and public previews of the five tunnel stations were held