AT LONG LAST, Loudoun County has agreed to sign on to Metrorail’s Silver Line extension to Dulles airport. Tuesday’s decision by the county’s governing Board of Supervisors — on a 5 to 4 vote cast hours before a crucial deadline — provides the linchpin that should allow construction to begin next year on the mega-project’s second and final stage, which will reach west to the airport and beyond, culminating with two stations in Loudoun.

The importance of Loudoun’s vote, for the county and the region, is huge. Had the county opted out of Dulles rail, it would have forced a back-to-the-drawing-board rewrite of the project’s legal, financial and environmental blueprints. That would have meant a lengthy, and possibly fatal, delay for an undertaking critical to the success of the airport and Northern Virginia.

By opting in, Loudoun, whose explosive growth over the last decade has transformed the western fringe of the metropolitan area, in effect decided to tighten its bonds with the region. In the process it will also juice economic development in the part of the county near the airport and contribute to ongoing efforts to ease some of Northern Virginia’s worst traffic congestion.

The board’s decision should have been a no-brainer, yet it was anything but. Polls, e-mails, public testimony and the virtually unanimous weight of the county’s business leaders all favored Loudoun’s participation. But a vocal and impassioned minority mounted an aggressive campaign to stop the project, vilifying Metro — sometimes in veiled racist terms — as a force for evil.

Some critics also had reasonable concerns, such as that Dulles rail would impose an unfair tax burden on Loudoun residents, including many who wouldn’t benefit by the project. In the end, the board sensibly addressed that by establishing special tax districts to ensure that most of the county’s share of the construction cost will be borne by landowners and developers — but very few homeowners — in the immediate vicinity of the new rail stations.

In the end, reason prevailed, with a crucial assist from Virginia Gov. Robert F. McDonnell (R), who mounted a concerted lobbying effort toward the all-Republican county board. Mr. McDonnell deserves credit for the outcome; Dulles rail, assuming the project is completed, will be an important part of his legacy in the state.

Still, the governor’s work isn’t done. Despite Richmond’s recent decision to commit another $150 million to the Silver Line, the state’s total contribution comes to $425 million — just 8 percent of the project’s $5.6 billion total cost. Earlier this year Mr. McDonnell considered advocating an additional $300 million contribution but backed down in the face of pushback from downstate lawmakers.

The governor now needs to mount an aggressive effort to secure additional state funding, both as a matter of equity and of self-interest. Doing so will ease the rate of increases paid by Dulles Toll Road commuters, who are currently on the hook for a disproportionate share of the project cost. And it will signal that Virginia, whose economy will be the Silver Line’s main beneficiary, grasps the project’s centrality to its own future.