Sound Transit Light Rail to Cross I-90 Floating Bridge

The I-90 floating bridges in the state of Washington carry the interstate across Lake Washington between Seattle and Mercer Island. Innovative engineering by Sound Transit will enable its light rail trains to travel across Lake Washington on the floating bridge when East Link opens in 2023 (see above image, courtesy of Sound Transit). The first-of-a-kind achievement involves the development of a “track bridge” that will compensate for six ranges of lake motion.  Popular Science Magazine recently named the technology one of the most important engineering innovations of 2017.

The flexing track connection uses earthquake science to move trains across the floating bridge.  The challenge was for the bridge to remain buoyant while 300-ton trains pass each other.  The most difficult task was adapting the rails to the movements of the bridge—there was no room for error.

The new rail type will allow the train to function at full speed.  Trains will travel along the same kinds of high-strength bearings that allow buildings and fixed bridges to flex in an earthquake.

The track bridges will be assembled in controlled conditions so that movements and weather will not interfere with their construction.  Before occupied by passengers, they will run empty for at least three months as track movements are documented.

Other options were not viable. The lake is too deep to accommodate a fixed bridge or a tunnel, and a train-only bridge was not cost-effective. John Sleavin, Sound Transit executive technical advisor and lead engineer on the project, stated (ENR December 6, 2017) that there were no precedents for engineering such a multimodal joint. “You had to go back to engineering principles.”

According to Sound Transit (“Crossing Lake Washington,” October 23, 2017), to safely transition light rail vehicles from the I-90 land bridge to the floating bridge, engineers and designers had to consider a number of factors, including six different ranges of motion, up and down, back and forth and side to side. Sound Transit designed and tested the track bridge where the rails rest on a series of bearings and plates that allow them to move with changing lake levels and bridge movements.

Two track bridge prototypes, along with two Link light rail vehicles, were shipped to the Transportation Technology Center in Pueblo, Colorado, for testing. The track bridges passed all critical tests with the ability to provide safe and comfortable light rail operations at planned speeds of up to 55 mph, the highest speed that Link operates.

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Testing of track bridge, Courtesy of Sound Transit

East Link will provide a 15-minute ride between downtown Bellevue and downtown Seattle when it opens in 2023.  Click here to read more and view the technology in action.

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