[40] The waters of Puget Sound and the surrounding region's navigable rivers were the primary transportation corridors for the indigenous Coast Salish peoples as well as later settlers who arrived in the 19th century.

[41] It waned in importance as railroads and streetcar systems were constructed around Puget Sound; these services, later supplemented by interurban trains, grew in the early 20th century to serve a growing number of passenger commuters.

[43][44] The first major proposal for a rapid transit system to serve Seattle and the surrounding region was drafted by urban planner and civil engineer Virgil Bogue in 1911 as part of a comprehensive plan.

[51] Metro and the Puget Sound Council of Governments, the inter-county metropolitan planning organization for the Seattle area, completed a study in 1986 to identify potential corridors for a modern light rail system.

[60][61] The board of directors for the Central Puget Sound Regional Transit Authority, the official name of the RTA, held its first meeting on September 17, 1993, at a former Washington State Department of Transportation office in Bellevue.

It included 69 miles (111 km) of light rail service that would be completed within 16 years with lines that would connect Downtown Seattle to Lynnwood in the north, Bellevue and Redmond to the east, and Tacoma to the south.

[66][67] The plan also called for a shorter timeline to launch a commuter rail system, which would use an existing 81 miles (130 km) of freight tracks from Lakewood to Everett, and an express bus network with eight routes.

[67] The proposal was supported by prominent elected officials, including incumbent governor Mike Lowry, and the "pro" campaign received funding from Boeing, Weyerhaeuser, local retailers The Bon Marché and Nordstrom, and engineering firms.

[67][71] The "no" campaign primarily comprised businessmen from the Eastside region led by mall developer Kemper Freeman;[71] it argued that the plan was too expensive to construct and would not address traffic congestion.

[73][74] In Everett, 83 percent of voters rejected the ballot measure, attributed to the opposition of local elected officials due to the lack of light rail service for the city in the first phase.

[77][78] The RTA was reorganized to reduce its spending by 60 percent and its 150-person staff was cut to 23 members; a new CEO was hired and Snohomish County Executive Bob Drewel was elected as board chair to represent a "clean break" from earlier transit planning.

[79][80] The new plan, named "Sound Move", was adopted by the RTA board in May 1996 and was placed on the November 1996 ballot;[76] its development included over 400 public meetings to receive community input.

[85][86] The RTA began expanding its staff and moved out of its shared space with Metro in July 1997;[87] its new headquarters occupied several floors at 1100 Second Avenue, a former bank building in Downtown Seattle.

[111] By 2005, Sound Transit had constructed several direct access ramps between bus hubs and HOV lanes on freeways, along with a total of 10,000 stalls at park and ride lots.

[115] Earlier meetings had criticized the use of surface sections through Tukwila and the Rainier Valley, where a more expensive tunnel was rejected, due to their potential effects on displacement and travel time.

The area's poor soils and other changes to the design led to a $680 million increase (equivalent to $1.14 billion in 2023 dollars)[46] in the estimated cost of the project, which drew criticism from local media and elected officials.

[114] A private audit commissioned by Sound Transit determined that the agency's financial estimates had been "overly optimistic", lacked adequate contingencies, and were drawn from insufficient data.

[123] CEO Bob White resigned and was replaced by Joni Earl, previously chief operating officer; she is credited with salvaging the light rail project and restoring public trust in Sound Transit.

[124][125] Earl, an accountant who had little transit experience but was a city manager and deputy county executive under Drewel, sought to make the agency more transparent and produce a more realistic budget for its projects.

[126][127] The inspector general's interim report, released in April 2001, criticized the FTA and Sound Transit for advancing in the grant review process without having a firm cost estimate, which had changed several times due to modifications to the preferred project.

[143] Sound Transit began forming its long-range plan in 2004, which would include a new funding package for the remainder of Central Link as well as other projects to expand the rail and bus network.

[192][193] The route of the Eastside light rail line, under the project name East Link, was approved in April 2013 alongside additional funding from the Bellevue city government to cover the costs of a downtown tunnel.

The draft plan proposed a 25-year program that would open new transit projects from 2028 to 2041 that would include all modes and ultimately extend Link light rail to a 108-mile (174 km) network from Everett to Tacoma.

[220][221] An attempt to restrict the motor vehicle excise tax collected for ST3 to $30 was launched by Tim Eyman and passed in 2019 as Initiative 976, which was later ruled unconstitutional by the Washington Supreme Court.

[227] The local onset of the COVID-19 pandemic in early 2020 led to a 67 percent decline in Sound Transit ridership by mid-March after remote work policies were enacted by major employers in the Seattle area.

[228][229] The agency halted its fare collection and enforcement for several months and reduced service in response to the decline in ridership and lack of available staff during the beginning of state-mandated lockdown measures.

[240][241] The 2 Line, formerly named East Link, was originally scheduled to open from Seattle to western Redmond in 2023, but was delayed a year by construction issues and a four-month strike by concrete delivery drivers.

[246] The opening of the Lynnwood Link Extension increased daily ridership on the 1 Line to an average of 90,000 on weekdays in November despite reliability issues that led to major service disruptions by the end of 2024.

[4] Under a provision of the state constitution, Sound Transit is limited to issuing debt that does not exceed 1.5 percent of the assessed land value within the district;[268][269] the final bond payments under the program are scheduled for 2068.

[282][286] The security officers also conducted fare enforcement on Link light rail and Sounder commuter trains, which included a citation with a $124 fine for non-paying passengers, until the program was suspended in 2020 due to accusations of discrimination.