FOR THE PAST couple of months, the handful of unelected officials in charge of building Metro’s $6 billion Silver Line extension to Dulles International Airport and beyond have pushed the idea that federal loans — not serious cost-cutting — will make the project’s numbers work. On Wednesday they got a harsh reality check.

It came in a meeting called by Transportation Secretary Ray LaHood, who convened project stakeholders, including members of the Metropolitan Washington Airports Authority, which is in charge of building the 23-mile extension. In response to suggestions from the airports board that the feds could shore up the Silver Line’s shaky finances with a huge loan, Mr. LaHood delivered an unusually blunt message, according to several people who attended the meeting. The federal government “is not a cash cow,” he said; it’s not going to ride to the project’s rescue.

Mr. LaHood is playing the critical role of mediator in an attempt to save this region’s most important public infrastructure project. As head of a small working group of officials from the airports, the state of Virginia and local governments, he set a goal of 30 days to resolve the impasse that threatens to derail the project.

His comment about the feds’ limited resources should have come as a bracing bit of truth for the airports authority board, which runs Dulles and Reagan National airports as well as the Dulles Toll Road. Two months ago, the board voted 9 to 4 in favor of building an underground Metro station at Dulles rather than a surface one, which would add $330 million to the cost of the Silver Line yet save passengers scarcely three minutes of travel time.

There is well-justified concern that the project’s soaring price will be borne largely by Dulles Toll Road drivers, who are on the hook for most of the Silver Line’s costs. In response, the airports board said it had applied for a transportation loan, which, because of flexible terms and low interest rates, could hold down toll increases in the long term.

Unfortunately, the airports’ loan request under the Transportation Infrastructure Finance and Innovation Act (TIFIA), like many of the board’s ideas, is magical thinking. The amount requested, $1.7 billion, exceeds the total TIFIA funds available annually for the entire nation(about $1 billion). And it’s more than twice the size of all five TIFIA loans made to transit projects combined over the past 13 years. It’s not going to happen — as Mr. LaHood and other officials have now made clear.

It’s time for the airports authority to get serious about getting the Silver Line built responsibly and affordably. Cost-cutting — starting with the underground boondoggle at Dulles — is the key to making the project work.