Light rail across I-90 floating bridge will be first of its kind; closures of HOV lanes start June 3

Sound Transit’s $3.7 billion project connecting light rail to the Eastside over the I-90 bridge gets started June 1. The project is part of the East Link rail service that will create 14 miles of rail connecting the Eastside to Seattle, part of that stretch will mean trains traveling over the I-90 floating bridge.

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Plans to add light rail to the I-90 bridge between Seattle and Mercer Island are underway. But the floating bridge moves with Lake Washington’s water level and must be kept buoyant despite the new weight of tracks, ties and trains, installed in the existing express lanes. (Steve Ringman/The Seattle Times)

The project was approved by voters for the Sound Transit 2 measure, bringing 14 miles of rail to the Eastside, including across I-90, which would make for a 15-minute commute from Bellevue into downtown Seattle.

The floating elements of the I-90 bridge made the engineering and design even more challenging.

“The bridge goes up and down, also when the wind blows the bridge will go slightly north or south, because it’s on anchor cables much like a boat will kind of move around. And, then as traffic loads, the bridge will also move a little left and right,” said Sleavin.

Engineers had to accommodate six degrees of motion.

The transition point between the fixed and floating structure of the bridge was a standout place to maneuver.

At this transition, the rails will rest on bearings and plates allowing movement with the changing lake and bridge conditions. Sound Transit partnered with a company in Tacoma to build two full-size track bridges and test them under simulated operating conditions at a transportation technology center in Pueblo, Colorado.

Sound Transit said it passed all the critical tests.

Trains ran hundreds of times over the test tracks, mimicking the force and movement it’ll experience during normal operations at a speed of 55 mph.

“It’s weight, it’s balance, it’s trim, it’s basically, it’s a 3,500-foot-long boat we’re driving across,” said Sleavin.

Sound Transit says in the rare event of a strong windstorm, with winds between 30 to 40 mph that generate strong waves, it could cause train service to be reduced.

Four car trains are projected to start running in 2023, “And those will be at 8 minute headways in the peak hours, each direction, so about every four minutes there will be a train crossing the I-90 bridge,” said Sleavin.

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