EVERETT — Two proposed light rail tunnels in Seattle have officials farther north worried that the higher costs could delay Link trains getting to Everett.

The Sound Transit Board will decide this week whether to study two underground alternatives for the West Seattle and Ballard extensions that could add $1 billion to the project.

The tunnels were not part of the ST3 package voters approved in 2016, which also will connect the light-rail system to Everett and Tacoma.

Sound Transit has said that paying for the tunnels in Seattle would require third-party funding and could delay the opening of that extension.

Snohomish County officials fear the additional costs and timeline could impact the Everett light rail extension set to open in 2036. The West Seattle and Ballard lines are scheduled to start operating in 2030 and 2035, respectively.

“It is going to be too hard to justify making major changes that could impact the scope and costs,” said Edmonds Mayor Dave Earling, who was on Sound Transit’s board when it was created in 1993. “When you expand scope you delay projects.”

“Of concern to me, and my colleagues, is that we need to be able to complete the spine,” he added.

A motion approved by the board in 1994 prioritized linking the four major centers of Everett, Seattle, Tacoma and Bellevue.

“Building the spine first was an early commitment,” said Dave Somers, Snohomish County executive and a member of the Sound Transit Board.

A final decision on which alignments to study is expected at Thursday’s board meeting. Several alternatives will be selected to study for each portion of the routes.

Using third-party funding has been brought up as a way to cover the extra cost, but where it would come from has not been identified.

Somers said he would only support studying tunnel routes if it is clearly understood that third-party funding is needed and Seattle pays for it.

He doesn’t want to see state or other regional transportation dollars used to go underground.

Seattle Mayor Jenny Durkan supports moving forward with the tunnel options, but has said the additional costs should not be shouldered just by Seattle.

The original plan for ST3 included an elevated route through West Seattle and movable bridge to Ballard.

West Seattle residents argue a tunnel into the Alaska Junction, a main thoroughfare in the neighborhood, would be the least disruptive and displace fewer residents. One proposal floated to pay for part of the tunnel is to eliminate one of the five stations on the West Seattle route.

A tunnel would not increase ridership, according to the transit agency.

If the tunnel alignment is ultimately approved for West Seattle it could add $700 million to the price of the project.

In Ballard, a fixed bridge and tunnel are alternatives being considered for the route, both of which would have less of an impact on the maritime industry at Fisherman’s Terminal compared to a lower bridge that would need to open, according to Sound Transit. These options are estimated to add $100 million to $350 million.

“It’s beyond the scope of what the voters approved and that becomes an issue on how it gets funded,” said Paul Roberts, an Everett councilmember and member of the Sound Transit board, referring to the proposed tunnels. “We need to honor and build what we said we are going to build.”

Other changes that would add costs are also being considered, including moving or burying deeper two other stations along the Seattle alignment.

If Sound Transit commits to more expensive projects in Seattle and there’s a downturn in the economy, it will put other projects at risk, Somers said.

The Snohomish County officials also said increasing costs could limit bonding capacity for future projects, especially those at the end of the lines in Pierce and Snohomish counties.

“I really strongly believe the package approved is the package we are authorized to move forward with,” Somers said.

Lizz Giordano: 425-374-4165; egiordano@heraldnet.com; Twitter: @lizzgior.