How to Build Train Tracks Across a Bridge That Won't Hold Still

Seattle is already home to one of the world’s largest floating bridges. Now Sound Transit is taking on another challenge: building steel tracks for trains on a bridge that floats and moves to the whims of the water and wind. The city is now moving forward on a multi-billion-dollar project to extend light rail from Seattle to Bellevue across the Interstate 90 floating bridge on Lake Washington.

A team that included engineers from Sound Transit, the University of Washington, and independent experts spent years working on the engineering challenges, according to John Sleavin, Sound Transit executive technical advisor and lead engineer on the project. They settled on a curved wing design from English engineer Andy Foan.

The I-90 bridge has a floating span of nearly 1 mile. That floating section—which can move up and down as much as 2 feet, move north and south based on wind, and even tilt because of traffic loads—connects to fixed bridges on either side. Getting a 300-ton light rail train across that accordion-style joint is no easy task. Foan’s concept includes a 43-foot curved platform that can move in such a way to keep the tracks safe

Sleavin says to think of the design akin to a three-legged stool. The tracks rest on a platform that has pivoting bearings connected to a hinge. “Two points define a line and three points define a plane,” he says. “No matter where the bridge rotates, if you have an item sitting on three points you will always find a plane.” No matter which direction the floating bridge rotates and pushes the hinge, the connections self-level based on the movement, supporting the track at the joint in a continuous alignment.

Foan says his joint maximizes the speed the train can travel while providing a unique solution that allows for safety. “Quite honestly, when Andy laid it out, everyone’s jaws dropped,” Sleavin says. “It was a little bit of a feeling of, we are not worthy. It was so simple and so obvious.”

The design is self-adjusting and self-leveling. The team added rail ties atop double pendulum bearings, as commonly found in earthquake readiness designs, to allow for motion under the platform while ensuring the ties remained at a constant distance apart

To make sure the concept was viable, Sound Transit worked up computer modeling. Then they did small-scale testing in a lab at the University of Washington. To top it off, they moved to full-scale testing at a site in Pueblo, Colorado. Next comes construction, testing, implementation, and, eventually, perhaps an expansion to the SR 520 floating bridge, the world’s longest, which is just one bridge north on Lake Washingt