Most people barely notice the hill that Tysons Corner is built on, even though, at about 515 feet above sea level, it looms as Fairfax County's highest natural summit. For them, the slope demands no more than a nudge at the accelerator.

But for the engineers designing a Metrorail line to the area's malls and offices, the hill at Tysons Corner presents a significant obstacle: Railroad tracks generally must be laid flat or at very slight inclines. Otherwise, the steel wheels lose traction.

"It's just a hill to most people, but for us, the hill in Tysons Corner is one of our biggest engineering challenges," said Dulles Project Director Sam Carnaggio.

In fact, the fate of the entire rail project turns at least in part on the $100 million conundrum posed by the slope at Tysons Corner.

The effort to extend Metrorail through Tysons, a kind of holy grail for Northern Virginia commuters, has reached a critical juncture.

By late August, project managers must cut the estimated $2.4 billion cost by more than 20 percent -- or else scuttle the idea, they said. And as engineers review cost-cutting design alterations, no single greater dilemma exists than that presented by the hill.

Engineers could excavate a mile-long relatively level tunnel through the hill. But that could be done only at a budget-busting cost of $132 million, cost estimators say, and project managers have identified the elimination of any underground passage as the largest single cost savings they can make.

Running tracks on or close to the ground would disrupt traffic too much.

So the alternative to the tunnel is to cross the hill on elevated tracks, but that creates its own problems. The aerial tracks and their supports could look like a concrete scar running through Fairfax County's "downtown," many fear. Making matters worse, engineering constraints could force the tracks to be built as high as 80 feet aboveground.

Tysons Corner is the capital area's second-largest job center and home to two regional malls, and the county's comprehensive land development plan has long envisioned a rail line there -- preferably underground.

"A tunnel rather than an elevated alignment is the preferred mode," according to the comprehensive plan. Aerial tracks would, among other things, "intrude visually."

The dire cost-cutting challenge for the rail project arose last month, when engineering firms reviewing the plan, which would extend the Metro system from West Falls Church through Tysons Corner, estimated that its price had risen 60 percent, to $2.4 billion -- way beyond what planners and public officials consider feasible.

Not only does the project lack the budget for $2.4 billion, planners said, but the construction effort also would flunk federal cost-effectiveness standards at that price.

"We're looking at everything to find savings," said Josh Sawislak, deputy project director for the Virginia Department of Rail and Public Transportation, which is leading the project.

Among the proposed cost-cutting measures are a reduction in the number of rail cars, a drop in the size of the train platforms and the elimination of some elevators and escalators at the station.

There are drawbacks to virtually every cut, however: Reducing the number of rail cars would reduce the number of trains running at rush hour, one of the key reasons for building the project; shrinking the size of platforms could create safety problems; and eliminating escalators and elevators could run afoul of the Americans With Disabilities Act.

Rejecting those other changes, however, leaves only the issue of the hill.

Aside from the former county landfill near Interstate 66 and West Ox Road, which rises to 567 feet above sea level, the Tysons hill ranks as Fairfax's highest elevation, county geographers said.

"It's a relatively flat county," said Charles Grymes, who teaches geography at George Mason University. "Unfortunately, railroads can't climb hills worth a hoot."

The largest single proposed cut -- $132 million -- involves elimination of the mile-long tunnel that was to have traversed the hill at Tysons Corner and included one underground station.

But completely eliminating that tunnel would raise railroad tracks as high as eight stories high

The reason for the height is that railroad tracks can safely operate at only slight inclines. Metro guidelines generally limit slopes to no more than 4 percent, meaning that the tracks can drop no more than 4 feet over 100 horizontal feet.

If aerial tracks were run from the top of the Tysons hill, the ground would slope down much faster than the train tracks could, leaving the tracks several stories in the air at times. Eventually, the track would catch up with the ground, but in the meantime it would create big gaps between the ground and the tracks.

Engineers call this "chasing grade."

Abandoning the tunnel configuration could cause other troubles as well.

The property owners in Tysons Corner, who have agreed to put up as much as $400 million in tax money toward the project, based their offer on the original configuration of the train route. That offer might be rescinded if the configuration changes drastically.

"To the extent that there is a significant change, the county runs the risk of having to go back to landowners in order to spend the tax money," said John McGranahan, a lawyer who organized the tax district.