Engineers discuss the technique used to construct a tunnel under Tysons Corner, Va., as part of the Dulles rail project, designed to link Metro service to Dulles Airport and Loudoun County.
Photo by Nikki Kahn (The Washington Post)

Engineers discuss the technique used to construct a tunnel under Tysons Corner, Va., as part of the Dulles rail project, designed to link Metro service to Dulles Airport and Loudoun County.

Photo by Nikki Kahn (The Washington Post)