Sound Transit launches world's first floating bridge rail line

Sound Transit has implemented a pioneering engineering solution on the Homer M. Hadley Memorial Bridge over Lake Washington, creating the world’s first passenger rail line to operate on a floating bridge structure.

Lake Washington, located between Seattle and the Eastside (Bellevue, Redmond), presented significant geographic challenges due to its 60-meter depth and thick layers of soft silt. These conditions made traditional fixed bridge pillars or tunnels technically and financially unfeasible. While floating bridges using hollow concrete pontoons have been used for vehicles since the mid-20th century, operating a 272-ton light rail train on a moving surface was previously considered impossible.

To manage the multi-dimensional movement of the floating bridge—which shifts due to waves, wind, seasonal water levels, and thermal expansion—engineers developed a specialized track system. This technology allows the rails to transition smoothly between fixed land-based sections and the floating bridge, preventing derailment or structural failure caused by the bridge's constant motion.