Every few minutes, a train now glides across Lake Washington carrying riders between Seattle and the Eastside — a trip that, until recently, required navigating some of the region’s most congested highways.

The journey feels routine. The infrastructure behind it is anything but. With the opening of Sound Transit’s Crosslake Connection, Seattle became home to the world’s first light-rail service operating across a floating bridge, completing a long-envisioned regional transit connection decades in the making.

The final segment of Sound Transit’s 2 Line, the Crosslake Connection links the Eastside to Seattle and connects the 2 Line to the 1 Line at International District/ Chinatown Station. Its opening completes a major commitment to the voter-approved Sound Transit 2 expansion program.

The milestone strengthens a regional light-rail network that now spans more than 63 miles and provides new access to jobs, education, healthcare and cultural destinations throughout the Puget Sound region.

“This project fulfills a vision that has been years in the making,” said Mark Holmes, HNTB vice president and group director of construction services and project controls. “The Crosslake Connection brings together multiple investments across the region to create a transportation system that functions as a connected network rather than a collection of individual projects.”

HNTB served as part of the construction management consultant team, providing resident engineers, inspectors, cost facilitators and other key personnel who supported Sound Transit throughout project delivery. The firm’s involvement builds on a long-standing partnership with Sound Transit that has included final design services for portions of the East Link Extension and project manager roles on the Lynnwood Link Extension.

Building on a bridge that moves

While riders may experience the Crosslake Connection as a smooth trip across the lake, delivering that experience required solving a challenge never before attempted anywhere in the world.

Unlike conventional bridge structures, the floating bridge rises and falls with changing lake levels and responds continuously to weather and environmental conditions. That movement required project teams to rethink how rail infrastructure could be designed, installed and maintained.

“The floating bridge introduced conditions that conventional rail projects do not encounter,” said Nikola Dukleski, HNTB resident engineer. “Without an established blueprint to follow, the team relied on close collaboration, rigorous testing and shared problem-solving to develop solutions tailored to the bridge.”

Many of those challenges centered on preserving the integrity of the existing bridge while adapting it for rail service. Washington State Department of Transportation, which owns the floating bridge, worked closely with Sound Transit and project partners throughout design and construction. The project team developed specialized track attachment systems that minimize impacts on the existing structure while accommodating the bridge’s constant movement.

“WSDOT leveraged its almost 90 years of floating bridge design, construction, operations and maintenance expertise in working with the Sound Transit teams and international subject matter experts to implement the unprecedented operations of both light-rail transit and vehicle traffic on a floating bridge” said John White, WSDOT Assistant Secretary for Urban Mobility, Access and Megaprograms.

Construction activities also had to be coordinated around limited weather windows, with some work restricted to specific seasons when conditions on the lake were most favorable.

Preserving infrastructure for generations

Beyond the visible rail infrastructure, the project introduced innovative systems designed to protect the floating bridge for decades to come.

A first-of-its-kind cathodic protection system helps shield the bridge’s concrete pontoons and steel anchoring cables from corrosion. Because electrified rail systems can generate stray electrical currents, engineers developed an active protection strategy to help preserve the bridge’s structural integrity. The effort required extensive calibration, testing and coordination among multiple agencies.

The project also required significant upgrades to tunnel fire and life-safety systems where new rail infrastructure interfaces with existing facilities. Working within legacy infrastructure introduced constraints rarely encountered on new construction projects and required extensive coordination among transportation agencies, emergency responders and regulatory partners.

From construction management and quality oversight to testing and commissioning, teams worked through years of technical complexity to ensure every component performed as intended.

“The Crosslake Connection reflects years of planning and coordination across agencies,” Holmes said. “Serving as part of the construction management team, our staff worked alongside Sound Transit and project partners in a highly integrated, one-team structure to support delivery of this critical investment.”

A connected region

As significant as the engineering achievement may be, the project’s greatest impact may be what it means for the people who use it.

The Crosslake Connection completes the final missing link between Seattle and the Eastside, transforming how residents move. What once required navigating traffic congestion, toll bridges and lengthy bus trips can now be accomplished through a reliable rail connection linking major destinations.

The connection also strengthens service throughout the broader network. According to Sound Transit, trains now arrive as frequently as every four minutes during peak periods through the busiest section of the system, improving capacity and reliability for riders beyond the immediate corridor.

“The completed connection changes how people experience distance,” said Catherine Hovell, HNTB project director. “Destinations that once seemed disconnected become part of the same daily routine. People can spend less time thinking about how to get somewhere and more time focused on where they want to go.”

For many riders, the benefits extend beyond shorter travel times.

The completed rail extension expands access to education, employment and regional destinations. Students and workers can reach key destinations more easily, and visitors enjoy greater mobility without being limited by parking availability or highway congestion.

The opening also represents more than the completion of a single project. Together with the East Link and Lynnwood Link extensions, the Crosslake Connection transforms years of transit investment into a more connected regional network. HNTB contributed across all three efforts, supporting project delivery on Crosslake while helping advance key segments that now function as part of a unified system.

For the teams who spent years designing and constructing the transit connection, opening day marked the culmination of a long and ambitious effort.

“The Crosslake Connection represents years of dedication and collaboration,” said Tony Raben, executive project director at Sound Transit. “Designing and constructing light rail across a floating bridge required innovation and partnership at every level. This project delivers a connection that strengthens mobility and will serve riders for generations.”

Today, trains glide across Lake Washington every few minutes, carrying riders between communities that once felt much farther apart. For passengers, the journey may feel effortless.

For the region, it represents a transportation milestone unlike any other in the world, one that finally brings together years of transit investment into a single, connected system.