Seattle is to begin work on the world’s first floating light rail line as part of a USD3.7bn project to build a 14-mile light rail corridor linking Seattle to the city of Bellevue, on the east side of Lake Washington.
The line will be built on the I-90 bridge, currently the world’s longest floating bridge. It is currently the route of a busy highway, supported in the water by 24 pontoons. Buoyancy helps the bridge to stay afloat; it is anchored on cables so that it can move slightly with shifts in wind or with changing traffic loads, but a weight such as that of a fully-loaded train could see the pontoons dip by eight inches.
The light rail line will accommodate for these shifts in movement with flexible tracks more reminiscent of earthquake engineering technology, allowing the track to ‘roll’ with any movement. Track joints would be fastened with a series of bearings and plates, called ‘track bridges’, featuring steel wings that respond by rising and falling to variations in the angle of the hinges. The tracks thereby curve smoothly and vertically. Parallel tracking is maintained thanks to steel bars that sit on pivotal bearings, keeping the bars stable when the hinges move.
Gravel will also be removed from the pontoons to maintain buoyancy despite the additional weight of trams.
Work will begin on 3 June in the centre express lanes of I-90, with passenger service set to begin in 2023.