Most people outside Washington state have never heard of Sound Transit 3 (ST3). But within urban development circles, it has become one of the most closely watched transit programs in the country, a real-world case study in megaproject governance.
ST3 is the expansion plan for public transit in the Puget Sound region and one of the most ambitious transit investments ever approved in the U.S. Originally projected to cost over $50 billion, the program links Seattle, Tacoma, Bellevue, Redmond, and dozens of other cities into a single regional network. Voters approved the plan in 2016, authorizing a 30-year buildout that will create roughly 116 miles of light rail.
Several elements of ST3 rival standalone transit systems elsewhere in the country. The Everett and Tacoma Dome extensions alone are longer than the entire light-rail networks in many mid-sized U.S. cities.
Much of the system’s cost and risk stems from its complexity, especially in dense urban areas where tunneled stations can cost $300–500 million each. The West Seattle–Ballard Link highlights these challenges, requiring deep-bore tunneling beneath waterways and crowded neighborhoods on a scale comparable to subway construction in New York or San Francisco.
The most pressing challenge, however, has been cost escalation. Earlier this year, Sound Transit reported significant cost growth across ST3 projects, pushing projected costs up by roughly $30 billion. Officials have scrambled to close the gap, including briefly considering the elimination of a second downtown Seattle transit tunnel as a last-resort cost-cutting measure. That idea was rejected last week, sharpening regional tensions over priorities and cost-sharing as Sound Transit prepares to release an updated system plan in 2026.
ST3 also underscores the challenge of delivering a single megaproject across multiple municipalities within a fragmented political system. The West Seattle–Ballard Link illustrates this clearly: repeated routing changes, shaped by neighborhood-level political pressures, have increased costs and delays. As a result, ST3 is frequently cited in debates over whether voter-approved infrastructure programs require stronger centralized decision-making.
Finally, ST3 is a widely referenced example of the “infrastructure–zoning gap,” in which major transit investments proceed without corresponding land-use reform. The Issaquah Link underscores this risk: Sound Transit can build rail, but cities along the corridor control zoning and limit housing density, weakening ridership and the ST3’s economic returns. For planners, ST3 is a cautionary case showing that transit projects can fall short when land-use policy fails to align with infrastructure spending.