the King County Courthouse and the King County Administration Building, as well as other major buildings, including Smith Tower, Columbia Center and the Alaska Building. From Pioneer Square, the tunnel travels down a 5.5% grade to cross under the Great Northern Tunnel at a 45-degree angle near the intersection of 4th Avenue South and South Washington Street, briefly descending below sea level, before turning cardinal south into the International District neighborhood. International District/Chinatown station, the southernmost tunnel station, is partially open-air and located immediately below a public plaza at Union Station. The station has connections to Amtrak and Sounder commuter rail at King Street Station a block to the west, accessible through the Weller Street Bridge, as well as the First Hill Streetcar on Jackson Street, stopping east of 5th Avenue South. Other attractions near the station include Lumen Field to the west and Uwajimaya a block to the southeast. South of the station, the light rail tracks and bus lanes were formerly separated by railway signals at an underground bus layover and staging area next to the tunnel comfort room for bus drivers. The southern portal of the tunnel is located under the intersection of Airport Way and 5th Avenue South at the western terminus of the former Interstate 90 express lanes for high-occupancy vehicles. Until bus operations ceased, southbound routes continued from the tunnel in separated lanes on the SODO Busway, while eastbound buses used a set of ramps that traveled onto the Interstate 90 express lanes. The eastbound ramps will be retrofitted for the East Link light rail service that begins in 2025 on the 2 Line. Service The Downtown Seattle Transit Tunnel is part of the "Third Avenue Transit Spine", the busiest transit corridor in Seattle, serving a combined average of 54,000 weekday riders with bus stops on the surface. The tunnel has a theoretical capacity of 40 trains per hour per direction with a minimum of 90-second headways, carrying 22,000 passengers per hour per direction, but is only able to carry 12,000 per hour per direction in 4-car light rail vehicles with current systems in place. Prior to the start of Link light rail service, the DSTT could serve up to 145 buses during the afternoon rush hour. , the DSTT carried 52,600 daily riders, of which 10,000 are on light rail. The tunnel carries a segment of the 1 Line, which runs from Northgate