Would it be feasible to shift light rail from I-90 to 520?

When King County Executive candidate Susan Hutchison proposed switching the Eastside light- rail route from Interstate 90 to the planned new 520 bridge earlier this week, you might have heard a collective groan from elected officials and transportation engineers.

A 520 light-rail route would provide a straight shot between the University of Washington and Microsoft, one of the region's largest employers. And it offers the chance to put rail on the new bridge.

But changing the Sound Transit route at this point presents such enormous engineering, financial and political challenges that several key officials said they considered the idea unrealistic. The 520 bridge had previously been considered, but I-90 was deemed a better route.

Making the change now would require shelving nearly $30 million invested in engineering work for the I-90 corridor, cause significant delays in planning for the 520 bridge, and likely force another public vote.

The biggest hurdle might be convincing elected officials from across the region — some already leery of reopening the debate about replacing the Alaskan Way Viaduct with a deep-bore tunnel — to stop a transportation megaproject already in progress.

"I cannot think of any compelling reason that we would write those costs off and start over," said Fred Butler, a member of the Sound Transit board and Issaquah City Council who has not endorsed anyone in the county- executive race. "She is where we were probably four to five years ago."

Hutchison raised the idea during a debate on Wednesday with her opponent, Metropolitan King County Council member Dow Constantine.

"There's no reason to take lanes out of I-90, which is the major commuting road from the Eastside to Seattle, and turn it over to light rail," she said.

In an interview yesterday, Hutchison said, "I find it hard to believe that an organization that has spent billions over what was promised to taxpayers would not want to find the best alternative, even if it meant switching gears or ideas midstream."

Constantine has ridiculed his opponent's proposal to switch rail corridors, contending that she's really trying to block light rail. Hutchison denies that, saying she's a supporter of light rail.

If Hutchison is elected and pushes the proposal, she will likely face a skeptical Sound Transit board.

Approving a switch in routes would require a vote of the 18-member Sound Transit board. (If elected county executive, Hutchison would get an automatic seat on the board and could appoint two other members by the end of 2009.)

Several board members said that before considering a switch, they would require the idea to be put up for a new public vote.

The I-90 configuration was approved as part of a $17.9 billion Sound Transit plan approved by voters in 2008 to extend light rail to Northgate, Federal Way and Redmond.

Paula Hammond, the state transportation secretary and Sound Transit board member, said "I was surprised it (the 520 proposal) came up. I don't understand it."

"The voters have decided. It makes it a bit moot."

Hutchison believes voters were really just approving a general endorsement of extending rail to the Eastside rather than of a specific route.

Switching the rail corridor to 520 would also delay planning for the new bridge by several years, Hammond said.

DOT expects to receive bids in November to design and build the pontoons for the new six-lane 520 span.

The bridge is being designed to eventually accommodate rail via an expandable middle section, but not immediately. Seattle mayoral candidate Mike McGinn said he would like the new 520 bridge to include rail from the beginning in addition to rail on the I-90 route.

Running the Sound Transit line along 520 was seriously considered in the late 1990s. But there would be engineering challenges to doing so now.

The new 520 span is designed to be several stories high, while the Sound Transit station at the UW stadium is 85 feet underground. Connecting the two could potentially require a costly and complicated tunnel.

A 520 rail line could also cause a light-rail bottleneck at the UW station, where trains coming from the east and north would squeeze through the tunnel under Capitol Hill, according to a Sound Transit engineering report.

The congestion would be significant enough that a second tunnel might be needed, according to Sound Transit.

"It makes some sense until you look at what it really would cost, and how it would connect," said House Transportation Chairwoman Judy Clibborn, D-Mercer Island, a Constantine supporter.

"This is what you get when you have people who haven't been involved" in regional transit planning, Clibborn said.

There would be more complications on the Eastside, where Sound Transit has spent years negotiating with municipalities. A rail line along the 520 corridor would require a dogleg or spur line to reach Bellevue.

Claudia Balducci, Bellevue's deputy mayor, said Bellevue encouraged downtown density with the I-90 rail corridor in mind.

"We like to say in Bellevue, we are a city in a park. If we can't get light rail right through downtown Bellevue, we'd be a city in a parking lot," said Balducci, who has also endorsed Constantine.

Jonathan Martin: 206-464-2605 or jmartin@seattletimes.com

Information in this article, originally published October 23, 2009 and corrected October 23, 2009, incorrectly identified Claudia Balducci as a Sound Transit board member. We regret the error.