With the federal government's formal commitment to an extension of Metrorail to Dulles International Airport, we are about to witness the most significant expansion in the national capital's transportation system since the original Metro lines were laid out. The politicians and planners who struggled for decades to create the new Metro line across Northern Virginia know that. More than a dozen of them grinned in triumph for a photo at the signing ceremony Tuesday as the federal government pledged $900 million for the line through Tysons Corner and out to Reston. But commuters will know it, too, as they drive across construction zones in Northern Virginia from now until 2013. Here's what's happening.