



MERCER ISLAND, Wash. – The clock is ticking on the East Side. Come 2023, Sound Transit promises to complete the East Link extension line through Mercer Island, Bellevue and Redmond.



But to get there, trains have to cross Lake Washington, and engineers had to create new technologies to go over the I-90 floating bridge.



The new rail line will have to move from a fixed portion of the bridge deck to one that floats -- that meant engineers had to come up with a safe way to allow the tracks to flex.



The rails will also have to deal with seasonal changes in the lake like water level changes and winds that can push waves.



“It’s amazing, it’s sort of Sound Transit’s moon shot,” said engineer John Sleavin. “It’s never been done before. Having been lucky enough to have been here, it’s been really exciting.”



The solution is called a track bridge, and it allows the tracks to pivot along the joints where the fixed bridge connects to the pontoons.



“This has never been done anywhere else in the world,” said Sleavin.



When complete there will be eight-track bridges, two on each side in either direction.



Also, the train tracks are not connected directly to the bridge deck; instead, they’re placed on special pads that float rails above the surface. There will be approximately 9,000 of them across the bridge deck when it’s complete.