IN STRIKING A BARGAIN last summer between Northern Virginia localities, the state of Virginia and the authority that runs Washington-area airports, Transportation Secretary Ray LaHood managed to trim hundreds of millions of dollars from the burgeoning cost of completing Metro’s Silver Line extension to Dulles International Airport. Now Mr. LaHood has closed the deal by nailing down financing for the second phase of the $6 billion job and, in the process, has protected Dulles Toll Road users from outlandish toll hikes.

Although the feds put up $900 million for the first half of the 23-mile rail extension — about a quarter of the cost — their contribution for Phase 2, to Dulles and beyond, is relatively minor in dollar terms. However, Mr. LaHood has pledged federal help that will enable the airports authority and Northern Virginia localities to borrow money on excellent terms, and he has pledged to seek further federal assistance as the project goes forward.

The deal also includes a pledge from the state of Virginia for $150 million, which will be devoted to holding down toll increases on the Dulles Toll Road. Tolls will still rise, since revenue from the road, which is controlled by the airports authority, is still slated to cover most of the cost of the rail project’s second half. But they won’t quadruple or quintuple in the next few years, as had been projected under some scenarios.

The accord also limits the financial exposure of Fairfax County, to which Mr. LaHood transferred responsibility for building a Metro station at Route 28, just east of the airport, and two big garages along the Silver Line route. The county hopes to enlist the aid of developers who will assume a major part of the cost of construction. But in case it cannot, the bill will be shared with the project’s other stakeholders. That’s only fair.

The transportation secretary agreed to put the Silver Line project under oversight of the Federal Transit Administration (FTA), although construction will still be managed by the airports authority. The FTA’s involvement should provide a calming effect on what has become an increasingly fraught relationship between the airports authority on the one hand, some of whose unelected board members have taken a cavalier approach to costs, and Northern Virginia localities and the state on the other, where elected officials are wary of any new burdens on their strapped constituents.

The Silver Line remains one of the nation’s most ambitious public infrastructure projects. Its success is critical not only for Dulles’s future but also for Northern Virginia’s (and therefore the state’s) continued economic vibrancy. That alone justifies the investment of time and effort made by Mr. LaHood and federal officials in rescuing what looked last summer to be a project teetering on the brink of implosion as a result of spiraling cost projections.