The head of the Federal Transit Administration has issued a stern warning on the Tysons tunnel, telling local congressmen that changes in scope and uncertainties in costs could halt federal approval of the expansion of Metrorail to Dulles.

The letter is the latest rebuke by the federal government on the proposed underground rail through Tysons Corner, which tunnel proponents hope will be part of the larger 23-mile Dulles Corridor Metrorail Project. The concept was officially abandoned in September after the FTA warned that the more expensive plan would jeopardize $900 million in federal funding. A local movement has since emerged to bring the tunnel back, arguing the planned aerial rail would be a detriment to the urban center.