The Massachusetts Dept. of Transportation on Nov. 17 announced the award of a $1.08-billion contract for the Green Line Extension. The 4.7-mile extension project through Somerville and Medford went to a joint venture of GLX Constructors, comprising Fluor Enterprises, the Middlesex Corp., Herzog Contracting Corp. and Balfour Beatty Infrastructure. Construction will include track infrastructure, seven station stops, platform canopies and an enhanced vehicle maintenance facility in Somerville. The award, which came in some $200 million less than the agency’s $1.3-billion cap, requires formal approval by the agency’s board later this month. Ballooning costs nearly killed the project in 2015. Construction is expected to begin next year and reach completion by the end of 2021.