Wearing a tie dotted with Metro emblems, Virginia Gov. Gerald L. Baliles rode in the cab of an Orange Line train yesterday as it sped along the subway system's newly opened, 9.1-mile extension to Vienna.

"Can I do that?" asked the governor after the subway's operator sounded the train's piercing horn.

Alas, the chief executive of the Old Dominion was told he could not blow the horn. Undaunted, Baliles shrugged off the rebuff and peppered the operator, Alvin Godfrey, with questions. "I would be interested in knowing how to drive it if the need arose," said the governor.

Baliles, who later termed his trip "impressive" and "faster than the road," was among dozens of politicians and thousands of Virginia residents who turned out for a giddy, exuberant opening day for four new Metro stations along the median of I-66.

Passengers crowded onto trains and jammed parking lots at East and West Falls Church, Dunn Loring and Vienna. They waved orange balloons and wore T-shirts emblazoned with giant Farecards. More than 2,000 people joined in four hours of speeches and ribbon-cutting ceremonies. They quizzed officials about bus service to the stations and tried out Farecard machines, Metro's often balky fare collection devices.

By 4 p.m., Metro officials said, they had handed out 26,000 tickets for free opening-day rides at the new stations. No major delays or other incidents were reported. "We are go. A-OK," said Fady P. Bassily, Metro's assistant general manager for rail service.

Families came for a Saturday outing on the Orange Line, many toting children in backpacks and strollers. They arrived from Arlington, Fairfax and Prince William counties and from Falls Church, Fairfax City and Manassas. When the first train left Vienna at 8 a.m., several hundred passengers were aboard.

"I wanted to be the first one to ride it," said Elizabeth Freeman, a Dunn Loring resident who plans to commute by subway to Crystal City. Before getting on the 8 o'clock train, she snapped a photo of her friend, Margaret Inglehart, in front of the subway. "It's a great way to travel," said Inglehart.

Baliles was not the only politician to show an almost childlike delight in the opening-day frolicking.

Falls Church Mayor Carol W. DeLong tied bright orange laces in her walking shoes for a stroll to the East Falls Church station, a short distance outside the city's limits in Arlington. "I earned my button saying 'I walk to Metro,' " said DeLong.

Arlington County Board Chairman Mary Margaret Whipple told an Orange Line story. " 'Puff, puff, chug, chug,' went the little orange train," Whipple began. " 'We think we can. We think we can,' said the little orange engine." At the end of her tale, Metro had completed its planned 103-mile system. The crowd cheered.

With the extension's opening, the Metro system includes 69.6 miles of tracks and 61 stations. "This is the last opening for three or four years. This is it," said Metro General Manager Carmen E. Turner.

If funds are available, the transit authority plans to complete three more sections in 1990 -- a Yellow Line spur to a Van Dorn Street station in Alexandria's West End, a Red Line extension to Montgomery County's Wheaton station and a Green Line segment linking Anacostia with Northwest Washington.

Transit officials said the size of yesterday's crowds appeared to reflect widespread popularity for the new Metro stations but did not provide a reliable gauge of the number of passengers who will use the stations on a normal weekday. Tomorrow will offer the first test, they said.

Metro officials previously have predicted that the number of one-way trips taken by riders using the new stations will rise to 24,000 a day by the fall. The parking lots at the four stations include more than 4,600 spaces, but officials have said they expect most lots to fill up regularly within a few months.

Yesterday, the sprawling lots at Vienna were packed, and cars circled in search of spaces. Bill Cornelius, a Vienna resident who plans to commute on the Orange Line to downtown Washington, found a space in less than five minutes. "Got lucky," he said. Admitted another patron, "Parked where I wasn't supposed to."

Fairfax City's bus system, called CUE, began serving the Vienna station, drawing some riders and many questions about its new routes. Changes in Metrobus routes to serve the new rail stations are scheduled for June 22. A van service to connect West Falls Church with the Tysons Corner area is to start June 16.

Many homeowners near the new stations have expressed alarm over prospects of intensive development and traffic problems near the stations. Terry Carroll, a wheelchair-bound resident, went to the ceremonies to criticize the West Falls Church station, which has no approaching sidewalks, as "inaccessible."

Most passengers took yesterday's opening in stride. "We bought in the Vienna area because of this," said Jon Hiller, who plans to commute by subway from his new home to a job in Rosslyn. "It will take me right in to where I work."