The Crosslake Connection - Sound Transit’s Link 2 Line Extension

The world’s first light rail crossing a floating bridge will transform regional mobility across Lake Washington in metropolitan Seattle.

Location

Seattle, Washington, USA

Client

Sound Transit

Architect

Station Architects: HEWITT; Track Final Design: DEA; Floating Bridge Retrofit Final Design: KPFF

Project Value

$700+ million

Project Status

Ongoing

Awards

2017 Popular Science Magazine: Best of What’s New | 2019 Post-Tensioning Institute Award of Excellence for Repair, Rehabilitation, and Strengthening: I-90 Homer Hadley Floating Bridge Retrofits | 2025 American Public Transportation Association Innovation Award: East Link Light Rail Expansion: I-90 Segment

When it opens, Sound Transit’s Crosslake Connection will complete the Link 2 Line, adding the final seven miles of light rail service between downtown Seattle and the Eastside communities of Bellevue and Redmond.

What makes this project a world-class achievement is the unprecedented integration of light rail infrastructure onto existing highway infrastructure, particularly the Homer M. Hadley Floating Bridge across Lake Washington. This bridge has carried westbound Interstate 90 into Seattle since 1989, and starting next year will carry both highway and light rail traffic.

We served as the prime engineering consultant for the Final Design of this remarkable segment, placing us and our partners at the forefront of several extraordinary infrastructure achievements.

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Engineering achievements

The installation of light rail track on the Homer M. Hadley Floating Bridge required innovative solutions to address the dynamic behavior of floating infrastructure. To ensure safe and smooth transitions between the bridge’s fixed and floating segments, we developed a patented “track bridge” system that maintains rail gauge and a smooth ride across each of the bridge’s four large structural expansion joints, even as the bridge flexes under traffic and environmental forces and rises and falls with seasonal lake level changes.

The WSP-led team also enhanced the floating bridge’s structural resilience by adding longitudinal compression via 20 new external post-tensioning tendons, each exceeding 3,000 feet in length. These tendons strengthen the bridge and mitigate stresses caused by wind/wave forces and by the train and highway loadings.

Critical to the safety and durability of the bridge structure is the implementation of a non-invasive rail attachment method that avoids drilling into the top of the bridge pontoons. This approach preserves the bridge’s integrity and complies with Washington State Department of Transportation requirements.

Development size

7 MI

Bridge length

1.2 MI

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Overcoming challenges

The 2 Line Crosslake Connection for Sound Transit demanded innovative solutions and rigorous coordination.

One critical challenge was minimizing potential ecological impacts to Lake Washington beneath the bridge’s fixed (non-floating) approach spans. Our bridge engineers developed a seismic retrofit strategy for those fixed spans that eliminated in-water construction, revising the seismic characteristics of the bridges in a way that increased their seismic resilience while saving cost and ensuring the surrounding ecosystem was protected throughout the build.

To validate the bridge’s readiness for full-speed light rail, engineers from the WSP team (including many staff from subconsultant KPFF) recently completed the first round of dynamic testing to confirm that the deflection and load performance of the floating span fell within the range predicted analytically during Final Design under moving light rail trains. These tests verified the bridge’s behavior and brought the project closer to full scale rail operations.

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Benefits and recognition

This project will deliver tremendous benefits to communities across the Puget Sound region. By connecting Seattle with Eastside cities, the light rail extension will significantly reduce travel times and improve access to jobs, schools and essential services, allowing for a more equitable and efficient transportation network.

The project also will enrich rider experience through thoughtfully designed stations that incorporate public art and architectural elements, reflecting local culture and history. These installations foster a sense of place and community pride.

Economically, the project will strengthen regional development by linking neighborhoods, major tech hubs, and commercial districts. Improved connectivity encourages growth, attracts talent, and supports long-term investment in surrounding neighborhoods.