COAXED ALONG BY Transportation Secretary Ray LaHood, the outlines of a deal to save Metro’s multibillion-dollar Silver Line extension to Dulles International Airport seem to be emerging. That’s encouraging. But the Silver Line isn’t out of the woods yet.

Mr. LaHood, who stepped in as a mediator this month, is prodding the Metropolitan Washington Airports Authority, which operates Dulles and National airports and is building the Silver Line, to cut the project’s runaway costs so it can be built at a price stakeholders can swallow.

Extending from Reston to Dulles and west into Loudoun County, the 13-mile second leg of the Silver Line has been plagued by skyrocketing price projections. At this point, completing the Silver Line probably means cutting costs on the second phase by 15 to 20 percent so that the price is at or below $3 billion.

Fortunately, there are a number of sensible ways to do that. Building an aboveground station at Dulles in place of the planned underground station would save $330 million. Downsizing Metro’s planned rail yard at the airport and changing financing arrangements for five planned parking garages at the new Metro stations would save another $250 million. Some $200 million more could be slashed by eliminating one of the 11 planned stations in the project’s second phase.

Even if an agreement on costs is struck, the Silver Line should remain under scrutiny as one of the nation’s costliest infrastructure projects. One focus should be the airport board’s insistence that any general contractor bidding on the project be bound by a pro-union labor agreement. Already, Ken Cuccinelli II, Virginia’s attorney general, has threatened to sue if the deal turns out to violate the state’s labor laws.

The airports board was pushed to adopt the “project labor agreement” by board member Dennis Martire, who, in his day job, is a senior official in the Laborers’ International Union of North America, which represents hundreds of thousands of construction workers. Mr. Martire had an obvious conflict of interest, as his union would be a direct and major beneficiary from a labor deal. He should have had the common sense to recuse himself from the decision, even if his vote did not technically violate the board’s narrowly drawn ethics policy.

There is serious debate about whether a labor pact would drive up expenses. The airports authority says a similar pact for the project’s first phase, under construction from Falls Church to Reston, has helped contain costs and enhance efficiency by ensuring a steady supply of workers and avoiding labor trouble. But that agreement was adopted voluntarily by the contractor. There are concerns that a mandatory labor agreement for the second phase could dampen competition and drive up costs by discouraging bids from some large contractors, and by imposing cumbersome union rules. In an era of belt-tightening, the airports authority must go the extra mile to ensure that the Silver Line’s construction is managed as frugally as possible.