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Sound Transit is gluing the new light rail tracks to the I-90 floating bridge. They will not be secured in the concrete.

The tracks are being secured by 9,000 small concrete blocks that hang underneath them. Those blocks are then glued to the ground — the bridge deck — to make sure the tracks don’t move.

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“It’s a non-shrink grout, essentially a glue,” deputy director for design, engineering and construction management John Sleavin said. “A super, super glue.”

Sleavin said the state and the federal governments were more than a little concerned about drilling extra holes, about 18,000 of them, in a floating bridge. The blocks also help dissipate any electrical current that escapes the tracks.

But is it safe?

“We have done extensive testing in the lab for the strength of this,” Sleavin said. “We’ve done testing on how much rail force we get when the trains are going through.聽 Technically, with the weight of the rail and the blocks, you could actually not even glue them down, and (the tracks) would sit there. If you think of railroad ties, they’re just sitting there in ballast.聽 They don’t walk away.聽 But we want to be extra, double sure that these don’t move.”

The biggest engineering challenge on is how to make the transition from the floating bridge to the fixed structures at either end. Lake Washington’s water level typically goes up and down about two feet each year. You have waves and wind pushing the deck back and forth, and there are the changing loads, based on what’s going over the bridge.

The existing bridge used simple accordion expansion joints, which work great for cars, but not so well for train tracks.

“Rubber-tired cars can drive across that joint as it moves fairly easily,” Sleavin said. “A train can’t because the rail is fixed. If it were to move, and you attached the rails directly to that joint the rails would snap.”

The team created a special train bridge to spread that movement out over a longer distance to make the transition.

“This has never been done anywhere else in the world,” he said. “We had to invent this system. When we started, we weren’t sure we could do it.”

But after years of computer modeling, testing at the University of Washington and even building a mile of line in Colorado to run trains across it, Sleavin said they know it will work.

“We tested our trains from walking speed up to 55 miles an hour, which is our maximum running speed, to prove that it works,” he said. “A lot of time and effort went into making sure this unique design, that’s never been done anywhere in the world, could accommodate this.”

Milestone reached for rail on I-90 bridge

The tracks are being laid from the center of the floating bridge to each side. Right now, workers are assembling the eastbound tracks.

Sleavin said the project is on schedule, and light rail to Bellevue should be rolling in 2023.