Sound Transit plans to extend light rail tracks along the Interstate 5 alignment to Kent and Federal Way rather than along Highway 99.

The Sound Transit Board of Directors voted unanimously to pick the I-5 route as the preferred alternative for the 7.6-mile route from SeaTac to Federal Way at a Thursday meeting in Seattle.

But the board couldn’t agree on where the agency should build a station in the Kent/Des Moines area, instead it approved amendments for staff to further study whether to put a station on the east side or west side of Pacific Highway South near Highline College in Des Moines.

City officials from Kent, Des Moines, SeaTac and Federal Way already had agreed that I-5 would be the preferred route for the more than $1 billion project. That consensus by the cities impressed the Sound Transit board, which is composed of elected officials from across the region and chaired by King County Executive Dow Constantine.

Sound Transit plans to expand light rail from the Angle Lake Station at South 200th in SeaTac, which opens in 2016, to Kent/Des Moines near South 240th Street by 2023 and then the full 7.6 miles to Federal Way near South 320th Street when more funding is secured.

Now that the agency has a preferred route, staff will prepare a final environmental impact statement (FEIS) for the preferred route with a final board decision to be made late next year.

King County Councilman Dave Upthegrove, a member of the Sound Transit board, proposed the amendment to further study where to put a station in the Kent/Des Moines area. Other stations are expected to be built at South 272nd Street near I-5 at the Star Lake Park & Ride lot and at South 320th Street in Federal Way.

“This directs staff to consider and analyze options between 30th Avenue South and Highway 99,” Upthegrove said the board meeting. “This broadens it to include land at the college. …This keeps that option on the table and continues the discussion.”

Highline College staff and students support a station on the west side of Pacific Highway South also known as Highway 99. They want the station as close to campus as possible to make it easy for students to use light rail.

Kent city officials prefer a station on the east side of the highway near 30th Avenue South because it is less disruptive to existing businesses or future development on Highway 99. Kent also wants a pedestrian overpass so students and others can easily cross Pacific Highway South.

Ben Wolters, city economic and community development director, told the board maybe the agency could build an overpass similar to the one it just opened near the University of Washington campus across Montlake Boulevard near its light rail station by Husky Stadium.

Wolters and others also pointed out the I-5 alternative at an estimated cost of $1.4 billion is about $350 million less than the Highway 99 route.

The board asked staff to further study potential to create a transit corridor along South 272nd Street to better connect Highway 99 to the 272nd Street station with bus, bicycle and pedestrian routes.

Sound Transit staff is expected to return to the board in a few months with more specific information about the proposed station locations.