Central Link construction was delayed because of funding issues and routing disputes, but began in November 2003 and was completed on July 18, 2009, for $2.4 billion.

Sound Transit plans to expand the Link light rail network to 116 miles (187 km) and 83 stations by 2044, using funding approved by voters in 2008 and 2016 ballot measures.

Some of these systems were consolidated into the Seattle Municipal Street Railway, a city-owned network established in 1918, while others remained under the operation of their private owners.

[4][5] Unsuccessful proposals for a rapid transit system to connect Seattle's neighborhoods, and later other cities in the region, also emerged during the peak and later decline of streetcar and interurban service, both of which were dismantled by 1941.

[10] Metro's transit tunnel opened for buses in September 1990 and included tracks and electrical systems that would be compatible for future conversion to light rail.

[23] During the late 1990s and early 2000s, debates raged over various issues surrounding the Central Link line, which would connect Sea-Tac to Seattle.

[36][37] The line connects Northgate, the University of Washington, and Downtown Seattle to the Rainier Valley and Seattle–Tacoma International Airport, using tunnels, elevated guideways, and surface-running sections.

[39][40] In 2019, the 1 Line carried over 25 million passengers and averaged nearly 80,000 on weekdays, making it the busiest transit route in the Seattle region.

Construction of the line, also known as the East Link Extension, was funded by the Sound Transit 2 ballot measure and began in 2016 at a cost of $3.7 billion.

The line is 4 miles (6.4 km) long[51] and has twelve stations that serve the University of Washington's Tacoma campus, museums, hospitals, and other facilities.

[52][53] It runs primarily in traffic lanes, sharing the Commerce Street transit mall with buses, and uses a mix of single and double-tracked sections.

[67] To comply with the Americans with Disabilities Act of 1990 (ADA), all Link stations include accessible seating, signage with Braille lettering, pathways, level boarding, and tactile paving on the edge of platforms.

[63][70] These stations have platforms that are 380 feet (120 m) long to accommodate four-car trainsets and are connected to entrance structures at street level by stairs, elevators, and escalators.

Washington state law requires station identifiers other than their names written in the Latin alphabet, so Arabic numerals were selected instead.

The Central Operations and Maintenance Facility (OMF) in SODO is used to store, maintain, and clean trains during off-peak and overnight hours.

[37][98] All Link vehicles allow for level boarding and feature accessible seating areas that can be folded up for wheelchair users.

[68] Link uses positive train control and train-to-wayside communication to prevent operators from exceeding the set speed limit for a given section.

[100][101] The Series 1 fleet of 62 cars, manufactured in Osaka and assembled in Everett, was divided into three orders beginning with a contract signed in 2003.

[108][109] The Series 2 fleet, retroactively designated the Siemens S700,[110] was commissioned in 2016 as part of a $642 million order to support the ST2 expansion program.

The vehicles were manufactured in Sacramento, California, and are the same length and width as the Series 1 fleet, but feature a wider aisle in the articulated section, improved passenger information displays, and larger windows.

[123] The expansions are primarily funded through local taxes passed by voters in a pair of multimodal transit ballot measures.

[124][125] Sound Transit 3 (ST3) was approved in 2016 and funded new extensions of Link that will open between 2024 and 2046, including projects in Pierce and Snohomish counties.

[123] The cost estimate for the largest project in the ST3 package, the West Seattle/Ballard Link Extension, increased by 50 percent between 2019 and 2021, reaching $12 billion due to higher property values and lower revenue amid the pandemic.

[127][128] A set of new delays, mainly affecting Sound Transit 2 projects, was announced in 2022 following a four-month regional strike by concrete truck drivers, as well as unexpected conditions discovered during work.

Upon completion of several planned extensions in the 2020s and 2030s, the 1 Line would run from Tacoma Dome Station to Downtown Seattle, where it would use a new tunnel, and continue northwest to Ballard.

[131] In addition to the new lines, three infill stations in Seattle are planned to be built by 2031 at Boeing Access Road, Graham, and Northeast 130th Street.

[131][135] The western half of the East Link Extension is being built in the median of Interstate 90, including a section on a floating bridge that would be first railway of its kind in the world.

[142][143] Some figures and dates are provisional due to quality control, geological risk, and labor issues that have caused delays in some projects.

Climate change activists also point out that compact development around light rail lines has been shown to result in reductions in residents' CO2 emissions, compared to more conventional suburban automotive commutes.

[153][page needed] Environmentalists, transportation groups and some affordable housing advocates have sought greater government regulatory support for transit-oriented development along Link light rail, and in 2009 a bill was introduced in the Washington State Legislature that would have raised allowable densities (as well as lowering parking requirements and easing some other regulations on development) to a minimum of 50 units per acre in station areas.