Seattle just built the world’s first light rail on a floating bridge

If you live in Seattle and work at Amazon or Meta in nearby Bellevue, you probably drive to work. But by the end of next month there will be another option for commuters: the world’s first light rail line running on a floating bridge.

Right now, drivers cross Lake Washington—the long lake between Seattle and eastern suburbs like Bellevue—use one of three floating bridges. Conventional bridges aren’t feasible because of the depth and width of the lake, which is why the bridges were originally built with pontoons instead. Adding a rail line to one of them meant that designers needed to innovate in multiple ways.

First, since the bridge doesn’t have columns like a typical bridge, it moves. “It’s like a ship that’s been anchored to the floor of the lake,” says Brian Holloway, deputy director of engineering oversight at Sound Transit, the local transit agency. Near each end of the bridge, where the floating section connects to fixed parts of the bridge over land, hinge-like expansion joints let the bridge move as the water level changes or wind and waves slightly shift the structure.

Driving over the bridge in a car, you don’t notice the changes as the expansion joints move. But “those geometric changes would have a very significant effect on rail,” says Matthew Barber, a supervising engineer working on the project at WSP. To make light rail feasible, engineers designed a new solution: “track bridges” that support a section of rail on a structure with bearings that let the bridge move freely while keeping the rail steady. “The rail bends in a very smooth way,” Barber says.

The bearings are normally used in seismic retrofits in buildings. “Almost all the pieces on the floating bridge are not unique,” says Holloway. “They’re just being assembled in a different way.”

Weight was another challenge, since the pontoons that float the bridge weren’t designed to hold light rail. To help with that, the design uses thousands of ultra-lightweight concrete blocks to support the rail, using a mix developed and tested in a partnership with the University of Washington. The rail itself is a little shorter and lighter than typical rail to save more weight. When the rail was installed—replacing a former carpool lane—the team also removed a heavy concrete barrier at the edge of the former lane. All of this meant that the bridge could handle the extra weight.

On a normal bridge, installing rail would normally involve drilling, but the team didn’t want to risk drilling into the pontoons, which have to stay watertight. Instead, they used a special high-strength adhesive to attach the concrete blocks to the bridge. Since the bridge hadn’t originally been designed to carry electric light rail, engineers had to also find a way to protect it from stray current that could potentially damage the structure. The design now has multiple redundant solutions to avoid that risk.

The setting is unusual, since floating bridges are only used in specific conditions. (Norway’s fjords, for example, could potentially also use floating bridges.) But it’s possible that the design solutions could eventually be replicated in some other areas, including another bridge across Lake Washington in Seattle.

Even beyond the floating bridge, the new seven-mile stretch of light rail—from downtown Seattle to the southern end of Bellevue—required several creative new solutions. That included finding a new way to strengthen an overpass for earthquake safety, and reusing part of a former bridge to create access to a new train station in one neighborhood. “Every inch of the seven miles has examples of never-been-done-before, creative, resourceful designs,” Barber says. (All of this should go unnoticed by users, like any good civil engineering.)

On a recent test ride, he says that going over the bridge “was some of the smoothest track I’ve ever experienced,” as a daily commuter on light rail. The test ride was at night, so there wasn’t much traffic on the neighboring highway.

But he imagined it at rush hour. Tens of thousands of people are expected to ride the train daily, eliminating an estimated 230,000 vehicle miles traveled per day. “It was cool to be cruising along next to the cars,” he says. “And I can anticipate that when this opens, there will be lots of commuters on the train who will be zooming past folks who are stuck in traffic in a very satisfied way.”