Still in the planning process the project concerns two separate light rail extensions part of the regional ST3 package that voters approved funding for in November 2016.
West Seattle to downtown is a 4.7 mile line from from downtown Seattle to West Seattle's Alaska Junction neighborhood with five stations. Ballard to downtown comprises another 7.1 miles of light rail service from downtown Seattle to Ballard, including a new downtown Seattle rail-only tunnel and nine stations.
According to local media, the proposed second tunnel, which would run under Salmon Bay, is not covered as part of the money from ST3. The project would require third-party money to fund the tunnels. The other options are a movable bridge and a high, fixed bridge.
