Seattle will start construction next month for the world's first light rail on a floating bridge. Sound Transit/YouTube

Seattle built some of the longest highway bridges that float on water. Soon, one of them will carry a train system, too.

Seattle already has the world’s longest floating bridge, and next month, it’s going one step further: building the world’s first floating light rail line.

The undertaking is part of a $3.7 billion project to build a light rail corridor linking Seattle to the city of Bellevue on the east side of Lake Washington by 2023. It’s a tricky endeavor; the floating bridge has to withstand the pressure of two pairs of 300-ton trains traveling at speeds of up to 55 mph. To make it happen, the transit agency Sound Transit is turning to cutting-edge earthquake technology. With 50,000 riders expected daily, there’s absolutely no room for mistakes, the Seattle Times reports:

A derailed train would sink 200 feet to the lake bed. If track components break or wear out, transit service would be halted for maintenance, or subjected to slowdowns.

Still, this isn’t a city that shies away from massive infrastructure ventures. Its replacement project for the Alaskan Way Viaduct introduced us to Bertha, the world’s largest tunnel boring machine, which emerged from its controversial years-long trek under downtown Seattle earlier this year. And as I reported in December, it’s also working on a new “flexible” bridge designed to ride out an earthquake with little to no damage.

Seattle is also home to four of the world’s longest floating bridges. One of them, the Homer M. Hadley Memorial Bridge, spanning more than 5,800 feet along Interstate 90, connects Seattle to nearby Mercer Island.* Come June, its two reversible HOV lanes will be replaced with railway tracks, and the rest of the bridge will be reinforced to sustain the added weight and movement. The light rail, according to the Times, will be 30 percent heavier than what the bridge is currently designed to withstand.

To grasp the complexity and the immense challenge ahead, it helps to understand how these bridges work. The I-90 bridge, as it’s better known, is essentially supported by more than two dozen massive pontoons—watertight concrete blocks filled with air. Buoyancy helps these structures stay afloat, and maintenance checks are strenuously done to prevent any cracks. According to the Times, some of the gravel within the pontoons will be removed to prevent the rail cars from throwing off the balance, thereby keeping buoyancy.

The more significant challenge is at the hinges that join the floating and fixed sections of the bridge together. Below the deck, pontoons are linked together, and steel cables anchor them to the lake bed, which helps keep them from violently wobbling during strong winds and fierce waves.

Still, the bridge moves. “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,” John Sleavin, deputy executive director of technical oversight for Sound Transit, told local news channel Q13 Fox. “And, then as traffic loads, the bridge will also move a little left and right.”

While barely noticeable to the motorist, the railway track has to accommodate for these motions to ensure a smooth ride. Here’s where the earthquake engineering comes in, the main idea being that the rail tracks should be flexible. Their solution came when an English track design specialist proposed fastening the joints with a series of bearings and plates, known as “track bridges.”

These track bridges—eight in total, each spanning 43 inches—feature steel “wings” that, when the angle of the hinges change, respond by rising and falling. That causes the railway tracks to curve vertically and smoothly. To keep the tracks parallel during this movement, they’re held in place by steel bars that sit on “pivotal bearings” that will keep those bars stable when the hinges move. You can see a detailed breakdown of the system by the Times, or watch the animation below:

The design is unprecedented, so it’s understandable that the project makes some people nervous. But engineers at Sound Transit have used computer models to run simulations and even built two full-size track bridges for testing—albeit in a controlled setting inside a Colorado transportation center. At one point in 2005, Sound Transit teamed up with the state transportation department and sent trucks filled with 148,000 pounds of concrete slabs down the I-90 bridge to test whether it could withstand the weight of a light rail system. The agency also says that after completion of the rail, engineers will test-run empty trains for at least three months.

With a multibillion-dollar price tag, is the project worth the cost and the risks? The project has long been given the green light by the state government, but it’s had its opponents, including developer Kemper Freeman, who argued that the HOV lanes should be reserved for motorists. He took his cause to court, only to have the state Supreme Court reject it.

Whether the project proves fruitful will depend on flawless execution—and on real ridership numbers matching the expected tens of thousands of passengers each day. It will also depend on whether it really cuts down on the volume of motor traffic, as it’s intended to. The project certainly plays to Sound Transit’s mission to complete one of the largest transit projects in the U.S. by doubling the city’s light rail system over the next 25 years.

And while the technology is innovative—“brilliant” at the Times puts it—the idea of a floating light rail has been in the works since in the ‘70s, when both the federal and local government agreed that one day, some form of public transit—either a bus or rail system—would run along the floating bridge.

*CORRECTION: This article originally misidentified the floating bridge that will carry the light rail.

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