The Downtown Seattle Transit Tunnel (DSTT), also referred to as the Metro Bus Tunnel, is a pair of tunnels for public transit that run north–south under 3rd Avenue through Downtown Seattle, Washington from 9th Avenue and Pike Street to 5th Avenue South and South Jackson Street. The double-track tunnel and its stations, with the exception of Convention Place, constitute the northernmost section of the Central Link light rail line, continuing south through the Rainier Valley to Seattle–Tacoma International Airport as part of Sound Transit's Link Light Rail network. All five of its stations are also served by buses from King County Metro and Sound Transit Express 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, having signed a joint-operating agreement after ownership was transferred back to King County in 2002. The Downtown Seattle Transit Tunnel is one of two rail-bus tunnels in the United States, alongside the Mount Washington Transit Tunnel in Pittsburgh, Pennsylvania. However, the DSTT is the only rail-bus tunnel in the United States with stations, as the Mount Washington Transit Tunnel lacks stations.