The 2 Line, also known as the East Link Extension, is a light rail line serving the Eastside region of the Seattle metropolitan area in the U.S. state of Washington. It is part of Sound Transit's Link light rail system and runs for 10 miles (16 km) in the cities of Bellevue and Redmond. The line has ten stations between South Bellevue and Downtown Redmond stations and its first section opened on April 27, 2024. The full line with service to Downtown Seattle, Mercer Island, Bellevue, and Redmond, is planned to open in early 2026 and span 18 miles (29 km). The 2 Line will continue through the Downtown Seattle Transit Tunnel and share stations with the 1 Line through to Lynnwood City Center station.

The East Link project was approved by voters in the 2008 Sound Transit 2 ballot measure, with construction costs projected at $3.7 billion. The line will use the Homer M. Hadley Memorial Bridge, one of the Interstate 90 floating bridges, which was constructed in 1989 with the intent to convert its reversible express lanes to light rail. Early transit plans from the 1960s proposed an Eastside rail system, but preliminary planning on the system did not begin until Sound Transit's formation in the early 1990s.

The proposed alignment of the East Link project was debated by the Bellevue city council in the early 2010s, with members split on two different routes south of Downtown Bellevue; city funding for the downtown segment's tunnel was also debated and ultimately included in the final agreement. The alignment was finalized in 2013, after more than two years of debate, and delayed the beginning of construction to 2016 and the completion of the project several years beyond 2021. The Seattle–Overlake section was scheduled to open in 2023, but was postponed due to construction issues on the floating bridge section that resulted in the replacement of its plinths. As a result, the first section of the 2 Line opened in 2024 with Bellevue–Overlake service; it was extended east into Downtown Redmond in May 2025. The full line will include the world's first railway constructed on a floating bridge and is expected to carry 50,000 daily riders by 2030.

The Eastside suburbs underwent rapid development into bedroom communities after the 1940 opening of the first Lake Washington floating bridge, which replaced a cross-lake ferry system as the main connection to Seattle. While private bus operators ran routes over the Lake Washington Floating Bridge from Seattle to Eastside towns, the municipal Seattle Transit System opted not to extend its routes. The 1963 opening of the Evergreen Point Floating Bridge fueled further growth, leading to traffic congestion on both bridges during peak periods. By 1965, more than 150,000 people lived on the Eastside; the King County government predicted in 1965 that up to 550,000 people would live in Eastside cities by 1990.

In the 1960s, the construction of a rapid transit system for the Seattle metropolitan area was explored by municipal and regional governments. The initial system, serving the city of Seattle, would be extended east to Bellevue via Mercer Island and an additional floating bridge in a later phase, to be built by 1990. The state government amended its plans for a parallel floating bridge to Mercer Island to include exclusive right of way for rapid transit. The Eastside section of the rapid transit system was expanded to include branches to Eastgate and the Bel-Red area, with provisions to extend the system to Redmond. Intermediate stations would be located at Rainier Avenue in Seattle, on Mercer Island, and in Downtown Bellevue. The proposal, funded with federal grants, was put to a public vote as part of the Forward Thrust referendums in 1968 and 1970, requiring a 60 percent majority in order to use increased property taxes. During both referendums, voters were unable to meet the required majority to approve the rapid transit plan; the first earned a simple majority, while the second failed due to local economic conditions.

Metro Transit was created by a 1972 referendum to operate a countywide bus system and resumed planning work for a Seattle–Eastside transit system to be built using the new Interstate 90 floating bridge. A transit element for the new floating bridge was requested by the Puget Sound Council of Governments and a group of municipal leaders from Seattle and the Eastside, seeking to avoid additional traffic. A memorandum of agreement signed by the Washington State Highway Commission, Metro Transit, and local governments in late 1976 designated the center two lanes of the eight-lane bridge for use by transit and Mercer Island residents, with possible conversion to a fixed-guideway system in the future. The Puget Sound Council of Governments (PSCOG), a regional planning organization, determined in a 1981 study that light rail would be a feasible way to relieve traffic on the Interstate 90 corridor and recommended that Metro include it in their long-term plan. A joint study by Metro and the PSCOG in 1986 explored several alternative routes for an Eastside light rail system, recommending a route across the Interstate 90 bridge and branching from Downtown Bellevue to Kirkland and Bothell, via the Eastside railway, and Downtown Redmond, via State Route 520. While the light rail plan was left unfunded, provisions were made to accommodate a future Eastside light rail connection in the Downtown Seattle bus tunnel that opened in 1990.

In 1990, the state legislature called for the creation of a regional transit authority to fund the construction of a light rail system serving the Seattle metropolitan area. A work group, known as the Joint Regional Policy Committee, drafted a transit plan in March 1993 that conceived of a 105-mile (169 km) rapid rail system with a line between Seattle, Bellevue, and Redmond served by through-trains from Snohomish County. The plan recommended a hybrid rail and bus system, with rail serving as the primary form of high-capacity transit on the Interstate 90 corridor. Later that year, the Central Puget Sound Regional Transit Authority, later renamed Sound Transit, was established and began planning a package of transit projects for a public referendum. The committee's plan was downsized to 69 miles (111 km) of light rail, including a line from Seattle to Bellevue and Overlake, and submitted to voters on March 14, 1995. The $6.7 billion transit package was rejected by voters, in part due to an opposition campaign funded by Bellevue developer Kemper Freeman. A smaller, $3.9 billion system was proposed the following year, with a light rail line serving Seattle and Seattle–Tacoma International Airport instead of a system spanning the entire metropolitan area; express buses from Seattle to the Eastside would be funded by the package in place of light rail service. The removal of the Eastside light rail line was opposed by politicians and business groups in the area, with prominent developers funding an opposition campaign. The package was approved by the Puget Sound region's voters on November 5, 1996, earning a majority of votes in Eastside cities. Express buses between Seattle and the Eastside, funded by Sound Transit, began operating in 1999.

In the late 1990s, Sound Transit and the Washington State Department of Transportation (WSDOT) co-led a study into congestion-relief measures on the State Route 520 corridor, including the use of light rail on either of the floating bridges. The final Trans-Lake Washington Study, published in 2002, recommended further development of a high-capacity fixed transit system on Interstate 90 to supplement a new floating bridge carrying State Route 520. While the Trans-Lake Washington Study was underway, Sound Transit formed a steering committee in 1998 to assess configuration options for Interstate 90 and its existing reversible express lane. The committee recommended the addition of high-occupancy vehicle lanes (HOV lanes) to both directions of Interstate 90 by narrowing the existing lanes, leaving the reversible lanes ready for future transit use. The lane conversion, coupled with additional HOV ramps on Mercer Island, was approved by Sound Transit, WSDOT, and the Federal Highway Administration (FHWA) in 2004. An amendment to the 1976 memorandum signed by Metro and municipal governments to build the Interstate 90 bridge was drafted and signed, authorizing the conversion of the reversible lanes for high-capacity transit.