Rendering courtesy of Sound Transit

Rendering courtesy of Sound Transit

Seattle’s Sound Transit is one step closer to the first-in-the-world achievement of operating light rail on a floating bridge as construction crews install the innovative track bridges that will enable light rail vehicles to transition onto the floating bridge, and the specialized track attachments that minimize weight and isolate stray current from the electric system that powers the trains.

The process of installing rail on the floating bridge has been underway for a year, but the planning and preparation has been several years in the making. Last summer crews completed post-tensioning of the bridge pontoons, a process that reinforced the concrete with high-strength tensioned strands. Post-tensioning enables the bridge to withstand stronger winds and higher waves, as well as strengthen it to carry light rail tracks and vehicles. With post-tensioning complete, crews began installing track on the bridge in late 2018.

Almost 9,000 specially engineered and constructed lightweight concrete blocks are being affixed to the bridge deck using a specialized epoxy called DexG. Rail is set on the blocks, with steel tie bars placed between them to maintain gauge, similar to standard railroad tie systems. Features of the concrete blocks also isolate stray current that could damage bridge structures.


To date, crews have installed one of a total of eight innovative track bridges. The track bridges were specially engineered to compensate for six ranges of lake motion to enable trains to safely travel from the fixed sections of the bridge to the floating section. All of the track bridges are scheduled to be in place by the end of this year.

When the Blue Line opens in 2023, riders will be able to travel from Mercer Island to the University of Washington in 20 minutes; from South Bellevue to Sea-Tac Airport in 50 minutes; and from Redmond Technology to Bellevue Downtown in 10 minutes.

Sound Transit is simultaneously working to extend light rail north, south, east, and west, opening new stations every few years to form a 116-mile regional system by 2041. The Northgate Link Extension adds three stations to the Red Line in 2021. In 2024 the Blue Line will extend from Overlake into downtown Redmond. Also in 2024, additional extensions will begin operating to Shoreline, Mountlake Terrace, Lynnwood, Kent/Des Moines, and Federal Way. Further light rail extensions are scheduled to reach West Seattle, Fife and Tacoma in 2030; Ballard in 2035; Paine Field and Everett in 2036; and South Kirkland and Issaquah in 2041.

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