Seattle is building the world's first floating passenger train

Seattle, Washington is no stranger to ambitious transportation projects. The city is already home to four of the world's longest floating bridges. (Unlike a traditional bridge, a floating one often uses massive pontoons — watertight concrete blocks filled with air — to support its road deck.) Earlier this year, Seattle also completed an earthquake-resistant bridge.

Now the city is embarking on an even more ambitious transport project. Local transit agency Sound Transit is build the world's first light rail (a passenger train designed for light traffic) — on a floating bridge over Lake Washington by 2023.

The bridge will include two pairs of 300-ton trains that will travel up to 55 mph. It's part of a larger $3.7 billion project to construct a light rail corridor that connects Seattle with Bellevue, Washington.

As The Seattle Times notes, there's not a lot of room for error with the project's floating component. Sound Transit is expecting 50,000 daily riders for the new rail. If a train were to go off the bridge's tracks, it would sink 200 feet to the bottom of the lake.

Like Seattle's other floating bridges, over two dozen giant pontoons will connect to make it buoyant, according toCityLab. Below the deck, steel cables will anchor the pontoons to the lake bed, which will protect the bridge from rocking during strong waves or wind. The city will also perform routine maintenance checks for cracks.

According to the Times, two of the project's goals to reduce car traffic and carbon emissions in Seattle. Sound Transit has said that it aims to doublethe city's light rail system over the next 25 years. If successful, it would be one of the largest transit projects in the United States.

Other cities around the US have similar efforts to increase their public transit options. Los Angeles's City Council recently approved the Mobility 2035 Plan, which aims to build a new network of bus-only lanes and bike lanes. Meanwhile, Phoenix, Arizona plans to extend its existing streetcar and rail systems in almost every direction.