READY OR NOT, a Metro train to Dulles International Airport seems to be leaving the station at increasing speed, with taxpayers in the dark about where they're being taken -- and for how much. It's a highly confidential mission: With no public input whatsoever, Virginia officials signed a contract June 11 with a private firm to engineer, design and build an extension of Metro to Tysons Corner and the Dulles corridor. The agreement came within 24 hours after the federal officials gave its blessing to Virginia to begin engineering work on a $1.5 billion stretch -- part of a $4 billion project, the third most expensive rail project in the country. This may be the way to go, but what about alternatives? How does anybody know if this is good deal or not?

As reported by The Post's Lyndsey Layton, the contract calls for Virginia to pay Dulles Transit Partners LLC $45.5 million for preliminary engineering for a 23-mile rail extension from West Falls Church to Route 772 in Loudoun County, estimated to take about 15 months. That is supposed to result in a more detailed plan that will include sharper estimates of construction costs. Then, we're told, Virginia and the firm would have to negotiate a separate agreement for final design and construction.

Karen J. Rae, director of the Virginia Department of Rail and Public Transportation, says the deal gives the state greater control, that the project will be completed faster and at a lower cost than if it were done by Metro. Maybe, but should taxpayers take it on faith? Ms. Rae says that Metro's slightly lower estimate of $42 million was too speculative and that the private firm's total package will prove more attractive. That's nice, but again, how does anybody footing the bills for all this -- state and local taxpayers and those who are about to pay higher fares to ride the rails -- know what to expect or demand?

Under the arrangement, Metro will oversee preliminary engineering work by the private firm. The Federal Transit Administration required that the state use Metro's expertise because the state agency has not managed construction of a major rail project. When Virginia came before the Metro board in April with a request that Metro agree to act in this capacity, Metro Board Chairman Robert Smith raised a good question. He noted that while he supports private investment in public transportation, he was concerned about Metro's liability in this project in case disputes arise over costs of changing any aspect of the project.

An even greater nagging question: Is rail the best way to go, or are supporters of rapid bus service right in saying an efficient bus line could move as many or more people at far less cost? Before it's too late, somebody should pull the emergency brake so the public can get a good look at the landscape.