1 Line trains continue north from the tunnel to Lynnwood City Center 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 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 tunnel was closed on September 24, 2005, for modification to accommodate both buses and Sound Transit's Central Link (now the 1 Line) light rail trains with shared lanes and platforms.

Until its closure in 2018, Convention Place was the only bus-exclusive station in the tunnel, and consisted of four sheltered platforms in a sunken, open-air layover space below street level.

[16] The area around the station is known as the Westlake Hub, with connections to the South Lake Union Streetcar and the Seattle Center Monorail as well as the King County Metro and Sound Transit buses.

[17] Leaving Westlake Station, the bored set of twin tunnels turn south under Century Square to follow 3rd Avenue and its transit mall through the central business district, parallel to the shoreline of Elliott Bay.

[52] The New Flyer buses, dubbed "tunnel buses" by King County Metro, were ordered in 2004 to replace a fleet of Breda dual-mode electric trolleybuses whose overhead wire was to be removed in the tunnel's renovation for light rail;[53] 59 of the dual-mode Breda coaches were converted into fully electric trolleybuses between 2004 and 2007 and moved to surface routes,[54][55] where they continued to operate for more than a decade before being fully replaced in 2016.

[58][59] The plan was supported by City Engineer Reginald H. Thomson and the Municipal League among others, but opposed by businesses fearing it would shift the commercial district further north, and by the three daily newspapers published in Seattle.

[70] A second attempt on May 19, 1970, with an adjusted $440 million local contribution (equivalent to $3.45 billion in 2023 dollars)[10] and $881 million federal grant (equivalent to $6.91 billion in 2023 dollars),[10] failed to pass with only 46% approval amid a local recession caused by layoffs at Boeing; the earmarked funds intended for the Forward Thrust rapid transit project were instead allocated to Atlanta, to build its rapid transit system.

The study suggested that a double-decked tunnel with automated guideway transit to complement bus service, running from Union Station to the Seattle Center, would be able to meet projected demand at an estimated cost of $450 million.

[86] The cost of the project drew criticism at public hearings for using a significant portion of Metro's capital budget, a total of $840 million from sales tax revenue approved by voters in 1980, as well as potential disruption to business during the cut-and-cover construction of the stations and Pine Street segment of the tunnel.

Metro tested a prototype Renault PER 180 dual-mode trolleybus in 1983, describing it as problematic after finding it exceeded freeway axle load limits by 2 short tons (1.8 t) and having to replace several parts after several mechanical failures.

[94][95] In March 1986, the federal government offered Metro a contract committing $195 million in UMTA funding toward the bus tunnel, requiring that a decision on whether to move forward with the project be made by December 31.

[7][107] The TBM, designed by Robbins Company of Kent, Washington, and built by Nicholson Manufacturing in Seattle, began digging the western tunnel from Union Station the following May.

[111] Work on the western tunnel was briefly interrupted in November, when the TBM hit an unexpectedly large pocket of loose sand under Madison Street that had to be stabilized with grout to prevent damage to the adjacent Seattle City Light building.

[120] Pine Street was briefly re-opened for the Christmas shopping season beginning November 1 at the request of downtown merchants, with a temporary surface laid over backfill for automobiles and pedestrians.

[130] Tunnel construction was finished on June 7, 1990, leaving Metro to test safety systems and train personnel for regular service to begin in September.

[139] Metro's ban on South African goods was stricter than the federal sanctions, which only prohibited importation of specific materials such as steel, coal, uranium and agricultural products, and was used by UMTA to threaten to pull its funding for the bus tunnel project.

[141] Kalberer, who received praise for his work on the project before the scandal, admitted that the failure to consult the Metro Council's tunnel subcommittee before signing off on the deal was his mistake.

[141][142] Before this discovery, in September 1987, African-American rights groups had forced tunnel contractor Atkinson/Dillingham to return 36 steel beams from South Africa used for temporary shoring at Pioneer Square station.

The paper's editorial board also requested that Metro expand tunnel hours to keep buses running for late-night workers and entertainment events held on weekends.

The authority, later named Sound Transit, gained voter approval in November 1996 to build a $3.9 billion system between Seattle–Tacoma International Airport, Downtown Seattle, and the University District.

[169] The two agencies signed an agreement in May 2000 that formally transferred ownership and responsibility for the tunnel to Sound Transit, in exchange for $195.6 million paid to King County Metro.

[181][182] In February 2004, Sound Transit announced that it would begin the closure and renovation of the bus tunnel in September 2005, two years earlier than scheduled, to save money and reduce construction delays.

[183] Construction of a stub tunnel under Pine Street near Convention Place station, to be used for light rail train storage and turnback, as well as a future extension to Capitol Hill, began in January 2005.

[189] During the closure, Metro found that average travel time through downtown increased only 11 percent during the afternoon peak period, and ridership on popular routes only dropped slightly.

[206] In 2017, Sound Transit and the Seattle Department of Transportation announced that the tunnel would become rail-only in 2019,[207] two years sooner than the planned switch in 2021 to coincide with the opening of the Northgate Link Extension.

Over a ten-week period that began in January 2020, four-car trains ran on a single track within the tunnel, with through-passengers transferring at Pioneer Square station via a temporary center platform, reducing frequency to 13–15 minutes.

[220] The full transfer of ownership was approved by Sound Transit on October 27, 2022, at no cost due to an agreement with Metro to pay off $87 million of remaining debt incurred from its construction.

Other tunnel work, including the replacement of 58 bond boxes for signaling between the tracks, was also undertaken during the project, which had single-track service on weekdays and a full closure of the downtown section on weekends.