It's rush hour, and the signs of crowding on Metro's Orange Line in Northern Virginia are unmistakable.

First comes the dash for a parking spot at the station. Then the passengers jostle for an empty seat. And then the delay when trains stack up as they reach the system's choke point, the tunnel under the Potomac River between Rosslyn and Foggy Bottom, which is big enough to accommodate only one track in each direction.

"It's insane," said Anita Tawil, an airline sales executive waiting to board a train recently at the West Falls Church Station. "I've been taking this line for a good 15 years, and there are more and more delays, especially in the last year. Being in transportation myself -- well, when we have a delay, we apologize."

Crowding and delays have long been an affliction of Northern Virginia's highways, but they increasingly are becoming a part of life for thousands of riders on the region's Metro lines as well.

If and when the proposed $4 billion Orange Line extension through Tysons Corner and Reston to Dulles International Airport is built, thousands of additional passengers could be packed onto trains heading into downtown Washington. A recent Metro study estimated 73,000 weekday passengers would ride the subway to and from stops on the extension to Reston.

The added burden raises questions about whether Metro can carry everyone on the Orange Line, especially because of the bottleneck at the Potomac River tunnel shared by Blue and Orange line trains from Virginia.

Transit officials such as Jim Hughes, Metro's planning director, said they are prepared.

By next year, he said, Metro will begin replacing six-car trains with eight-car trains, adding a third more space to each train.

Metro is also working on a plan to ease the crunch at the Potomac River tunnel by diverting every other rush-hour Blue Line train to the Potomac River bridge that carries the Yellow Line between Northern Virginia and the District. The rerouted trains would skip some Blue Line stations, inconveniencing passengers waiting at those stops while making the trip faster for others. Most significantly, however, the diversion would free up critical tunnel capacity for Orange Line demands.

"We're at the point where we are putting five pounds of cars through a four-pound tunnel," Hughes said. The tunnel opened in 1977 to Blue Line trains and two years later began serving trains from the first Orange Line stations in Northern Virginia.

The tunnel is best suited for about 26 trains an hour; at the peak of the rush most mornings, 29 trains an hour pass through, Hughes said.

The extra trains mean delays up and down the line, largely because when one train is held up, it means that several others are likely to be late as well.

"There's not adequate spacing," Hughes said. "If we got back to 26, we could cut back on the bumper effect, which is something like the rubbernecking effects on a highway."

Not surprisingly, money is crucial to the proposed solutions, and some question whether Metro, which is already facing a budget deficit, can keep up with growing ridership -- much of it from Fairfax County's three Orange Line stops -- along with the planned extension to Dulles. The first phase of the project, an extension between West Falls Church and Wiehle Avenue in Reston, is scheduled to open in 2011.

Even without the Orange Line extension, Metrorail ridership was projected to double between 2000 and 2025, officials said.

William Vincent, a former U.S. Department of Transportation official who served during the Clinton administration, is among those who wonder whether Metro's train system can keep up with the projected growth.

He said, for example, that the Dulles rail extension would create an estimated $110 million annual deficit for Metro. The transit agency is expecting a $40 million shortfall in next year's budget.

"Metro cannot even afford the system that it has now," said Vincent, who advocates expanded express bus service instead of rail to Dulles. "Until these problems are fixed, and until we find a way to manage Metro in a sustainable and affordable way, I think it is reckless in the extreme to propose a massive expansion of the system."

Under the first phase of the Orange Line extension to Dulles, 11.6 miles of track and five stations would be added, extending from West Falls Church through Tysons Corner to Wiehle Avenue. Virginia received permission in June to start engineering work on the proposal, a necessary step before federal money can be set aside for construction.

Of the 73,000 projected weekday riders on the extension to Reston, transit officials said in the recent study, the largest category of passengers -- 23,500 -- would be reverse commuters, or people from the District and its closest Virginia suburbs traveling between home and their jobs in the Tysons Corner area. These riders are unlikely to exacerbate crowding, because they would be going opposite the main flow.

But about 18,300 would be riders heading in and out of downtown Washington, making crowding -- dubbed "the Orange Crush" -- only more severe.

"Unless they do something, that will be a disaster," said Jim Mikesell, a federal government economist and Orange Line rider from McLean.

Aside from the eight-car trains, finding a way to get more Orange Line trains through the tunnel is considered essential by transit officials.

During the busiest hour on weekday mornings, 19 Orange Line trains and 10 Blue Line trains pass through the portal between Rosslyn and Foggy Bottom.

Under the train diversion plan, every other Blue Line train from the Franconia-Springfield Station would split from the Blue Line at the Pentagon and cross the river and Washington Channel on the bridge that carries the Yellow Line.

Riders on those trains would miss several Blue Line stops and wind up with a direct lift to L'Enfant Plaza, the first Yellow Line stop in the District. Those who have to get to Rosslyn or one of the other omitted stops might have to wait a few extra minutes for the next train.

"Those riders that are being impacted obviously won't like it," Hughes said. "Some will end up with about half the service. For some, it will make their trip better."

Even with the eight-car trains and the diversion plan, the system could be full again by 2025, possibly making the construction of another tunnel a necessity. Building such a tunnel could cost more than $1 billion.

"I think the tunnel is one of the major hidden costs of Dulles rail," Vincent said. "They [Metro] stated very clearly that after 2025, service reliability cannot be maintained without it."

For now, however, several riders said they would cope and hope for the best. Metro still is preferable to driving, they said.

"It's a minor inconvenience," said Bob Burnett, creative director for a television production company, who rides the Orange Line between West Falls Church and Farragut West. "I'd rather stand and move than sit [in a car] and do nothing."

Commuters catch a weekday morning train at the West Falls Church Metro station. Since 1995, the station has experienced a 62 percent increase in the average number of riders yearly.The choke point, Metro officials say, is the tunnel under the Potomac River shared by the Orange and Blue lines, which is big enough to accommodate only one track in each direction.Crowds head into and out of the West Falls Church Station during morning rush hour. A planned extension to Reston is expected to add more than 73,000 weekday passengers to the Orange Line.