Engineering a world-first electrified rail connection across a floating bridge

Transforming cross-lake travel in the growing Seattle region

Residents and commuters across Seattle, Bellevue, and Redmond have long faced mounting travel challenges as the population in the region continues to grow. Travel to these areas required crossing Lake Washington via the congested I-90 and SR-250 bridges, with slow, unreliable, and increasingly unsustainable modes of travel.  

To address these pressures and provide a modern, high-capacity alternative, Sound Transit advanced the East Link System Extension as a transformative investment in regional mobility. With 14 miles of electrified light rail and 10 new stations, the extension is designed to move up to 50,000 daily riders with reliable, frequent service, including a 10-minute trip between Bellevue and Redmond. The project not only relieves congestion, but expands equitable access to jobs, education, and community services in Mercer Island, Bellevue, Overlake, Redmond, and the broader Puget Sound region, supporting long-term growth and enhancing quality of life for tens of thousands of travelers. 

Powering seamless integration across complex systems 

As part of Northwest Transit Systems Partners (a Mott MacDonald and STV joint venture), we worked with Sound Transit to deliver construction management consultancy (CMC) services, including construction management, contract oversight, commercial management, risk tracking, and project controls for system expansion.

Our team provided technical oversight of traction power, overhead contact system, train control, communications, and SCADA to verify compliance with design requirements and industry standards. We coordinated interfaces among systems, trackwork, structures, and stations to minimize construction risk, prevent rework, and maintain efficient sequencing.

Image credit: Sound Transit

Image credit: Sound Transit

Image credit: Sound Transit

A challenging and defining feature of the East Link corridor is the I-90 floating bridge, the world’s first floating structure designed to carry an electrified light rail train. We led systems construction management for the bridge, including stray current protection, wind and wave impact monitoring, and dynamic live-wire testing under operational conditions to safely operate rail on a dynamically moving structure.

We managed quality assurance processes through test plan review, field inspections, factory and site acceptance testing, and commissioning documentation verification. We also led all systems integrated testing efforts, supporting the transition into operations, rail activation, system safety certification, and operational readiness processes for the agency. Throughout the project, we coordinated closely with multiple stakeholders to align with safety, schedule, and performance objectives.

Lasting improvements in regional connectivity

With the opening of the East Link System Extension in March 2026, Sound Transit delivers a safer, more reliable cross-lake travel experience, providing consistent light rail service across the I-90 floating bridge and seamless connections among Seattle, Bellevue, and Redmond.

The extension offers a high-capacity, grade-separated alternative to congested highways and peak-period roadway delays. Beyond improving mobility, the project strengthens long-term network resilience and advances environmental performance through electrified rail service that reduces vehicle miles traveled and lowers regional emissions, helping Puget Sound communities move toward a more sustainable future.

Advancing light rail across Seattle

Mott MacDonald has been supporting Sound Transit in expanding its regional light rail network through major projects such as the Northgate, Lynnwood, Federal Way, West, and Ballard Link Extensions. Across these programs, we provide project management support, design build management services, construction management consultancy, systems engineering and assurance services. Together, these projects deliver fast, reliable light rail connecting communities across the Puget Sound region.