THIS PAGE HAS generally supported the plan to build a 23-mile Metrorail link through Tysons Corner to Dulles International Airport and points farther west, despite misgivings about the project's staggering cost and the fact that it may do little or nothing to ease traffic. We have argued that Dulles rail -- the nation's third-largest planned rail transit project -- holds the promise of lending focus, coherence and better access to Tysons, an economic dynamo whose 115,000 jobs make it the region's second downtown. But two recent developments have intensified our misgivings about the project's cost and invite closer public scrutiny and public engagement.

The first was the news last month that the first of Dulles rail's two phases, which would run from West Falls Church through Tysons and west to Reston, would cost $2.4 billion rather than the previous estimate of $1.5 billion. That revision, a 60 percent increase, has shattered the federal, state and local funding assumptions on which the venture is based and thrown everyone involved with it for a loop. For the past several weeks, engineers have scrambled to devise ways by which the new estimated cost might be shaved, say, 20 percent, possibly by running an unsightly rail bridge through Tysons instead of an expensive tunnel, or by deviating from (translation: downgrading) Metrorail's usual construction or design standards. Nonetheless, it is worth bearing in mind that since the project is split into two phases, it could conceivably end up costing $5 billion or more.

Then came the report by The Post's Peter Whoriskey last week that the Dulles rail project was the subject of a recent legislative gift, courtesy of Sen. John W. Warner (R-Va.), that allows it to pass a test for federal funding it would otherwise have flunked. Mr. Warner inserted an exemption into the transportation bill that cleared Congress last week enabling the project to remain eligible for hundreds of millions of dollars in federal funding even though it merits only a "medium-low" rating for cost-effectiveness -- the second-lowest of five categories. In other words, Dulles rail is starting to look like a vanity project, kept alive by its powerful friends despite the growing realization that most similarly rated projects around the country would never see the light of day.

Given what's now known about Dulles rail's likely cost, taxpayers are entitled to a full airing of the project's pros and cons -- and alternatives including innovative rapid bus service and road improvements. If there are acceptable revisions that would trim the price, those should also be given the maximum public exposure.

The benefits of Dulles rail as a spur to development in Northern Virginia still strike us as persuasive. But with $5 billion at stake, it is more important to consider the venture closely than to let its already considerable momentum and powerful political allies sweep away all reasonable doubts.