Construction is underway on the first phase of the Green Line Extension project, according to an announcement from the state Department of Transportation. Preliminary work includes the reconstruction of two rail bridges — one over Harvard Street in Medford and one over Medford Street in Somerville — and the demolition of an MBTA-owned building on Water Street in East Cambridge. The first phase, estimated to cost $12.9 million, is expected to be completed sometime in 2014, according to the Department of Transportation documents. The extension project will stretch the Green Line through Somerville to a terminal stop in Medford, creating six new stops. Planners have split the project into four phases, with the final work expected to be completed by 2020.