The approval is a critical step toward obtaining the necessary $900 million in federal funding for the 23-mile extension, which will be built in two phases.
The proposal now heads to U.S. Transportation Secretary Mary Peters, the Office of Management and Budget and Congress for final approval.
Gordon Hickey, a spokesman for Gov. Tim Kaine, said the governor was pleased with the FTA's decision but cautioned "it's too early to celebrate."
The future of Dulles rail looked grim nearly a year ago when the FTA unexpectedly said the project did not meet federal cost-efficiency standards.
The project's first segment will run to Reston and a second segment will stretch to Dulles.
(Copyright 2008 by The Associated Press. All Rights Reserved.)