The 12-mile rail line is part of a 23-mile Metrorail service extension to Dulles International Airport and Loudoun County.
The first phase of the project includes five new stations, a commuter parking garage, the procurement of 64 rail cars and additional capacity at the West Falls Church rail yard.
US Federal Transit Administration (FTA) deputy administrator Therese McMillan said: "With continued population growth expected over the next 20 years, .the Silver Line is an excellent investment in the transportation future of the Washington region
"This project will expand capacity for one of our nation’s largest transit agencies, accommodate thousands of additional riders for years to come and offer a much-needed alternative to traffic congestion in Northern Virginia."
The DoT has provided a total of $975m in funding while the remaining sum is being financed by state and local funding sources.
Being built by the Metropolitan Washington Airports Authority, the Silver Line will be operated by the Washington Metropolitan Area Transit Authority and the extension is expected to serve roughly 85,700 passengers a day by 2030.
Image: Unused concrete pier along WMATA K Route tracks for Silver Line bridge. Photo: courtesy of Ben Schumin.