The Sound Transit board unanimously endorsed new preferred light-rail routes for Bellevue on Thursday, including supporting a downtown tunnel.

The board also changed its preference for the controversial light-rail route in South Bellevue, passing on a Bellevue City Council recommendation for a route parallel to I-90 that would run through Mercer Slough and head north on the BNSF Railway.The board instead chose a more direct line into downtown along Bellevue Way and 112th Avenue Southeast on the eastern border of the Surrey Downs neighborhood.

"It's not the final milestone, but today is a major milestone and a culmination of a long year of joint work," said board member Claudia Balducci, also a Bellevue councilmember.

Board members said a financial pledge from the Bellevue City Council was an important factor in their support for the costly tunnel, estimated to add $320 million in costs to the East Link project and delay opening by a year. The Bellevue council voted Monday on a funding strategy to help find up to $150 million for the tunnel, although city and transit officials still need to work out a final funding strategy. If they can't, the board also approved a surface route that could be built instead.

"We've given ourselves an alternative we know we can afford if the tunnel doesn't turn out," said board member Fred Butler, of Issaquah. "This is a fair, reasonable and prudent course of action for the board to take."

But Mayor Don Davidson and Deputy Mayor Conrad Lee both reiterated at the meeting that the majority of the City Council does not support a light rail route along Bellevue Way and 112th Avenue.

Davidson favors the I-90 light-rail route, but on Monday, was a key vote on the divided council in favor of the general financial strategy to pay for a tunnel.

After Thursday's vote, Davidson said he felt "OK" about how everything had turned out. But he said he was still concerned about the preferred route for South Bellevue.

"There's plenty of time," he said. "We have a lot of questions."

Thursday's vote focuses the agency's engineering work on the preferred routes, but the transit agency is still studying environmental impacts for all of the routes proposed in an initial environmental impact statement plus four downtown routes added earlier this year. A final decision on Bellevue light-rail routes is expected next year.

"This is a significant measurement regarding our partnership and where we're going, but we have a long way to go," said board chair and Snohomish County Executive Aaron Reardon.

SOUND TRANSIT

The Sound Transit board endorsed light-rail routes in Bellevue that would swing up Bellevue Way and 112th Avenue Southeast and include a tunnel downtown.