The Metrorail extension through Tysons Corner has burned through more than 70 percent of its contingency fund. With two years of construction remaining, the jurisdictions paying for the line — and Dulles Toll Road drivers — could be on the hook for millions in possible cost overruns, transportation experts say.

The project, the first phase of Metrorail’s extension to Dulles International Airport and Loudoun County, began with $312.3 million allotted for unforeseen expenses two years ago. As of June, that fund had dwindled to $89.7 million.

That shrinkage feeds doubts about whether managers will be able to finish the project on time and on budget.

“That is certainly the goal, but there are challenges that remain,” said Patrick A. Nowakowski, the executive director of the Dulles rail project.

If the project goes over budget, the partners — Fairfax and Loudoun counties and the Metropolitan Washington Airports Authority — will have to make up a portion of the difference on the $2.8 billion project. Toll road drivers can also expect to pay more.

Airports authority board members said they are keeping a close watch on overall spending, which has totaled $1.4 billion so far.

“It is something we’re paying close attention to,” said Robert Brown, who chairs the airports authority board’s finance committee. “We’re watching what gets used because we don’t want to be over budget.”

The chairman of the Loudoun County Board of Supervisors, Scott K. York (I), said that if the project went over budget, all the partners would share the burden.

“We’re always concerned that something could go over budget,” York said. “They’ve told us that they were still well within the contingency.”

The chairman of the Fairfax County Board of Supervisors, Sharon Bulova (D), said that with 50 percent of the construction complete, she expects that those items that would have consumed large amounts of the contingency fund are mostly done.

“What I’m being told now from [the airports authority] and county staff is that they feel the project appears to be within budget and the contingency should not be used up and we will not be required to put in more money,” she said.

Fairfax monitors the project on a weekly basis, Bulova said. “I’m not losing any sleep yet, but I’m certainly paying attention,” she said.

Metro’s biggest expansion

The rail project, the biggest expansion of Metrorail since the system was built more than 35 years ago, has been fraught with controversy over its cost. Most recently the focus has been on whether the station at Dulles International Airport should be above or below ground.

The first phase of the 23-mile line, which is expected to be completed by December 2013, stretches 11.5 miles, branching off the Orange Line, running through Tysons Corner and terminating at Wiehle Avenue in Reston. Construction on that phase started in March 2009.

Nowakowski, who works for the airports authority, the entity responsible for building the Dulles rail line, said the most expensive parts of the first phase are done. A few items remain to bid, including pavers for station platforms, pedestrian bridges over roadways and the shifting of highway lanes back to their original locations.

Nowakowski said he is saying no to additional requests for project changes, watching staffing levels and “looking for opportunities to save money.”

Charles Snelling, the chairman of the airports authority board, said he is “very satisfied” that the project’s costs have been well managed.

“Most of the real troublesome stuff is behind us,” he said. “It is much more likely than not that we will be on target and on budget. We’re managing very tightly.”

But transportation experts are doubtful the project will stay within its cost estimates.

The original Metro system was estimated to cost $2.5 billion in 1969, but it came in at $3.8 billion — not counting inflation, according to Zachary M. Schrag, associate professor of history at George Mason University and author of “The Great Society Subway,” a history of Metro.

“It would be somewhat surprising for a major rail transit project to be completed on budget,” Schrag said. “Most major projects of any kind go over budget, that includes road projects, weapons systems, space programs, stadiums.”

Typically, overruns hit because it is hard to predict the cost of such expenses as materials and the relocation of utilities in a construction area, Schrag said. “It is kind of a vicious spiral where people low-ball the estimates to get their project approved,” he said.

Shared responsibility

The project’s financing required the agreement of several jurisdictions, and if the project goes over budget, they are responsible for making up part of the difference. Under the agreement, the fixed shares that Fairfax, Loudoun and the airports authority are obligated to pay total 25 percent.

If there are overruns, Snelling said, the partners “have to ante up their shares. It is our job to manage around the hazards to make sure that doesn’t happen.”

If Fairfax needed to pay more, Bulova said, the county would identify areas where it could raise additional funds. Possible sources include the general fund, bond funds or money from a commercial and an industrial tax whose revenue must be used for transportation projects.

But the percentage of the cost borne by Dulles Toll Road revenue also rises as the cost rises, according to the airports authority’s Brown.

That could mean higher tolls for drivers. In 2009, the authority adopted a three-year schedule of increases, with the next one — 25 cents at the main toll plaza — scheduled to take effect Jan. 1.

And then there’s Phase 2 of the rail project. Preliminary engineering is underway for the second leg, which will run to Dulles Airport and Loudoun, but local, state and federal officials are still hashing out how to pay for it.

One reason the contingency fund for Phase 1 has dwindled is that some work hasn’t gone as smoothly or as cheaply as hoped or planned. For starters, changes in the project design and related delays cost $20.2 million, Nowakowski said.

Construction was supposed to begin in 2007 but became bogged down in debates over funding, cost cutting and whether the rail line should run underground through Tysons Corner. The project “had to jump through a lot of hoops,” Nowakowski said. “It was killed, reborn, killed, reborn. To rebirth it, there is a lot of work involved in that.”

One of the biggest sources of cost overruns was moving utility lines, Nowakowski said. The airports authority has worked with 21 utility companies to move telephone, water, gas, electric and cable lines. The total cost was $44.5 million more than originally planned.

What Metro wanted

Metro, which will have ultimate responsibility for operating and maintaining the line, had changes of its own, changes the transit agency said were needed for safety and reliability. Those added $49.3 million.

The transit agency wanted two additional electrical substations along the first phase so trains would run reliably even if a substation lost power. The two substations and a system to monitor substations pushed costs $28 million over budget.

Another price increase came from having to redesign the West Falls Church yard to make it larger, to accommodate more rail cars. The cost was $5.7 million more than budgeted. Upgrading an emergency warning light system that alerts train operators when a track worker is on the line cost an extra $7.5 million. Switching from wood ties for the track to concrete ties cost $3.3 million more.

Metro spokesman Dan Stessel said some of the changes were necessary to ensure safety, reliability and “consistency with the rest of the system.”

Other items turned out cheaper than expected. The escalator and elevator bids for the first phase came in at $3 million less than expected. Some track work came in at $9.8 million less. And the project didn’t have to pay for a $27 million parking garage at Wiehle Avenue because a developer and Fairfax County are building it.

“There’s a few that came in our favor, but there’s a whole lot that didn’t,” Nowakowski said. “It goes with building a project of this size.”