Sound Transit says it has figured out how to run trains across the Interstate 90 floating bridge without being derailed by wind and waves.
The unorthodox design might require slower train speeds, officials said, and concrete must be removed from the surface to reduce the weight of the bridge across Lake Washington. It would be the world's first floating span to carry passenger trains.
Last week Sound Transit's governing board picked rail over express buses as its preferred mode of high-capacity transit across the lake.
The section is part of a proposed $3.9 billion line from Seattle's Chinatown International District to Bellevue, the Microsoft campus and downtown Redmond, to be completed in phases between 2015 and 2027.
The rail line won't be built unless voters pass a regional tax increase for more transit next year, along with a companion measure for highways in King, Snohomish and Pierce counties.
A state-appointed Expert Review Panel raised questions about rail on I-90 in March, but on Thursday, Chairman Mike Meyers, a civil-engineering professor at Georgia Tech, said that, based on current information, there appear to be no problems that would prevent trains on the bridge.
But panelist Alan Kiepper, retired president of New York City Transit, said heavy trains would load the span to 97 percent of its intended capacity. He even mentioned the original Tacoma Narrows Bridge, which collapsed in 1940 in a notorious case of hubris by engineers.
"Let me say, 'I have great concerns about the use of the bridge for light rail, and probably buses, and 97 percent is not a comfortable figure for me,' " Kiepper said.
The trickiest spot is the so-called "transition joint," where floating pontoons meet the fixed roadway near shore. The floating span can drift 2 feet east-to-west, and 5 feet up-and-down, as water conditions change.
Sound Transit would handle that by building overlapping rails on both sides of the joints. Similar rail designs exist on a cable-supported Vancouver, B.C., SkyTrain bridge and a suspension bridge in Lisbon, Portugal, said project manager Don Billen.
Another challenge is the weight of a four-car train. To guarantee buoyancy, a part of the concrete deck would be scraped off and replaced with a lightweight pavement. A concrete guardrail would be replaced by a steel-cable fence.
In September tests, the state Department of Transportation ran trucks loaded with 296 tons of concrete across the bridge to simulate train traffic.
Patrick Clarke, state structures engineer, said that with the Narrows debacle in WSDOT's history, he insisted on the live test, to check for the undulating motions that doomed "Galloping Gertie" in 1940.
"We didn't find any," he said.
Mike Lindblom: 206-515-5631