Passenger trains cross Lake Washington on floating bridge

A train carrying passengers rolls out from Seattle toward Bellevue, its 600,000-pound mass settling onto concrete pontoons on Lake Washington. The crossing is now operating on Interstate 90, on a bridge that moves with wind and waves rather than standing on conventional pillars. Sound Transit and the Washington State Department of Transportation had to make a railway work on top of a shifting structure.

The lake is a 200-foot-deep trough, and its bottom is covered with soft silt that makes traditional bridge supports impossible. Engineers reinforced the floating structure with a million feet of steel cable inside the pontoons and joined ten concrete blocks into one rigid barge. They also removed heavy gravel, used lightweight concrete and applied enough jacking force to make the bridge deck shrink three inches. A stalled train could reportedly have pushed the bridge down by eight inches without those measures.

Movement created the second problem. Lake Washington’s water level rises and falls two feet every year, while wind and waves keep pushing the bridge. Engineers placed track sections over expansion joints on 43-foot-long steel platforms supported by flexing seismic bearings. The continuous rails can bend as the bridge shifts, while the source says passengers experience a smooth ride.

Electric trains brought another risk: stray current can corrode steel and internal rebar when it cannot dissipate into soil. Engineers covered 9,000 rail support blocks in waterproof rubber and submerged hundreds of sacrificial anodes, designed to corrode instead of the bridge. A digital twin on shore receives live sensor data, including cable stress and pontoon depth measured to a tenth of an inch.

So what changes in practice? Seattle and Bellevue now have a passenger rail connection across a body of water where conventional pillars could not be used. The system is not presented as an easy construction success: the project ran six years late. But when the inaugural train reached the first pontoon, the structure dipped one inch, better than even the most optimistic computer models predicted.