The board of the Sonoma-Marin Area Rail Transit system awarded a $68.7 million contract to an Oakland firm on Wednesday for the next major phase of construction, primarily for track work in Marin County but including a new Petaluma River bridge and bike paths in both counties.

Board members awarded the contract to employee-owned Shimmick Construction, the lowest of three bidders seeking the work; competing bids ranged as high as $82.5 million.

The contract includes construction of track from Civic Center Drive to the downtown station in San Rafael; strengthening of the Puerto Suello Tunnel in San Rafael; and integration of a vehicular traffic control system in downtown San Rafael.

It includes replacement of the existing Haystack Bridge over the Petaluma River; construction of a new operations and maintenance facility; and finishing work on station platforms all along the line.

It also includes construction of more than three miles of bike paths next to the tracks in both counties.

The regional commuter rail system hopes to begin testing trains on the tracks next year and launch commercial service in 2016.