Rails need to stay straight, but floating bridges bob and sway with the water beneath. Not a good match? Not a problem for Sound Transit’s new project. On this East Link Bridge—which will be completed in 2023—steel platforms and flexible bearings will let light-rail tracks stay in line. By 2030, 50,000 commuters a day will ride 148,000-pound trains at full speed across the water from Seattle to Mercer Island, Washington.