Sen. Mark R. Warner (D-Va.) on Wednesday waded into the debate over the rising cost of the Metrorail extension to Dulles International Airport and Loudoun County. In a letter to the Metropolitan Washington Airports Authority, which manages the multibillion-dollar Metro project, Warner said he could no longer sit on the sidelines and called on the board to “recognize the realities of the current economic and fiscal environment in which we operate.”

“The clock continues to tick and I feel strongly that a resolution is needed sooner than later in order to ensure that this project continues toward its completion,” he wrote.

Warner said he “strongly encourages” the board to trim the current $3.5 billion cost of the second phase of the project to $3 billion.

A long list of local, state and federal officials from Northern Virginia, who are helping to pay for the project, have called on the board to reverse its vote in favor of an underground Metro station at Dulles. Opponents prefer a less expensive above-ground station that is not as close to the terminal.

In his letter Wednesday, Warner listed several possibilities for reducing the cost, including switching from an underground station to an above-ground location. But he did not dictate a path to achieving cost savings.

Warner is close to airports authority board member Mame Reiley, the former political director of his gubernatorial political action committee, who is a leading champion for the underground station. But the senator is also a member of the so-called Gang of Six in the Senate that has tried to find a bipartisan compromise for reducing the nation’s deficit.

Airports authority board members and elected officials are in the midst of negotiating cost reductions in discussions being mediated by U.S. Transportation Secretary Ray LaHood.

When asked about Warner’s letter, Reiley said she agreed with the senator’s call for “shared sacrifice,” but did not back away from the underground location.

“We can all get there,” she said, “and I think we can actually get there and save the underground station.”