SEATTLE — During a contentious meeting Thursday, the Sound Transit board voted 12-5 to continue to study options, including tunnels, for future light-rail lines to West Seattle and Ballard.

If they’re ultimately included, the underground segments could add an estimated $1 billion to the project while raising concerns about slowing Link light rail’s arrival to places such as Everett.

The tunnels were identified as one of two preferred alternatives. That means they’re defined by Sound Transit as a preference among designs being studied. The language for the tunnel routes included the caveat “with third-party funding.”

“The magnitude of what we are taking on has potential weak spots in scope, in timing and costs, because we don’t know those things yet,” said Edmonds Mayor Dave Earling, a Sound Transit Board member, during the meeting. “I still worry how the hell we are going to get to Tacoma and Everett.”

All three Snohomish County officials who sit on the Sound Transit board voted against the motion. Two Pierce County leaders joined them.

The tunnels were not part of the $54 billion ST3 package voters approved in 2016, which also will connect the light-rail system to Everett and Tacoma. The plan included an elevated route into West Seattle’s Alaska Junction and a movable bridge over Salmon Bay to Ballard.

The other preferred alternative identified for West Seattle was an elevated alignment that was very similar to what was part of the ST3 plan. In Ballard, a high-fixed bridge was designated as a second preferred alternative. A movable bridge will also be studied. The preferred options for Ballard seek to minimize conflicts with maritime traffic on the Ship Canal.

In an effort to speed up construction of ST3 projects and meet deadlines set in the package, Sound Transit wanted to identify a preferred alternative earlier in the process than was done in the past.

“Although we eliminated some of the alternatives… we still have quite a number on the table,” said Snohomish County Executive Dave Somers during the meeting.

Snohomish County officials worry if the scope and cost of the work increases with the inclusion of tunnel routes, it will impact projects further down the line. They have continually said a priority should be completing the light rail spine that connects Everett, Seattle, Bellevue and Tacoma.

“We will fight every step of the way for the regional system,” said Paul Roberts, an Everett councilmember and Sound Transit board member after the meeting.

The transit agency has said that third-party funding, which has yet to be identified, would be needed to pay for the underground routes. The change could also delay the opening of the extensions.

Additional funding for the underground portions must be paid for by Seattle, Somers said. He doesn’t want state or other regional transportation dollars used to cover added costs.

Some 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.

In 2020, the board will make a final decision on the alignments, whether to go underground or elevated. The board includes 10 members from King County, four from Pierce County, three from Snohomish County and the secretary of the Washington State Department of Transportation.

“We now have a path,” Earling said after the vote. “Obviously Snohomish County didn’t completely agree with the path.”

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