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Workers installing blocks and rail for Sound Transit on the I-90 floating bridge. (YouTube screen grab)

It’s already a pretty cool engineering marvel that the Seattle region has floating bridges carrying cars, buses and bikes across Lake Washington. Now we’re learning more about the technology that will enable Sound Transit to add trains to the mix on one of the structures.

Calling it a “first-in-the-world achievement,” Sound Transit said Tuesday that it is a step closer to operating light rail on the Homer M. Hadley (I-90) floating bridge thanks to innovative track attachments and track bridges.

The tech and engineering materials and processes are shown off in a new video released by the agency, below. Here are a few highlights:

  • Crews already completed post-tensioning of the bridge pontoons last summer, a process that reinforced the concrete with high-strength tensioned strands. This allows the bridge to withstand stronger winds and higher waves, while also strengthening it to carry light rail tracks and vehicles.
  • Track installation began in late 2018, and almost 9,000 specially engineered and constructed lightweight concrete blocks are being affixed to the bridge. Workers aren’t drilling holes in the bridge, instead they’re using a specialized epoxy called DexG — kind of like “super super glue.” Rail is set on the blocks, which also feature a plastic drip cap and a spongy layer called corkelast, designed to prevent stray electrical current which could damage bridge structures.
  • Specially engineered and meticulously tested track bridges are also being installed to compensate for six ranges of lake motion. This enables trains to safely travel from the fixed sections of the bridge to the floating section.

“To develop something unique, it’s sort of the whole fun of what engineering’s about,” Sound Transit’s John Sleavin said in the video. “The end product of this is going to be high-capacity rail serving the entire region for years to come.”

According to Sound Transit, the Blue Line is slated to open 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.

With rail extending in all directions, the plan is for a 116-mile regional system by 2041.

Eastside tech workers, at Microsoft in particular, will be watching for extension of rail from Overlake into downtown Redmond, including a station that could drop thousands of people off at the software giant’s campus every morning.

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