Four years ago, 80-year old Laurel Adams stopped driving her bright red 1960 Volkswsagen bug and began to rely on taxicabs and her nephew as her main means of getting everywhere. But yesterday, she put on her black felt hat, good camel coat, and fresh white gloves and descended into the subway.She was both apprehensive and thrilled about the prospect of her first Metro ride.

"Oh dear, it really does go underground, doesn't it?" she asked, hesitating at the entrance to the new Benning Road station. Three minutes later, on the train heading toward the Addison Road station in Prince George's County, Adams was smiling broadly and her dark brown eyes shone with delight. Metro had gained a convert.

"I didn't tell anyone I was going to do this today," said Adams, a retired school teacher who lives on Benning Road, NE. "My family doesn't understand about old ladies and adventure.Why, this train is just tremendous. How soon will I be able to ride down to Garfinckel's on it?"

Adams was one of hundreds of District of Columbia and suburban Maryland residents who sampled the city's rapid transit system first hand yesterday during the ceremonial opening of the Addison Road extensin of Metro's Blue Line.

The three new stations -- Benning Road at Benning Road NE and East Capitol Street in the District; Capitol Heights, in Prince George's County on Southern Avenue at East Capitol Street and Addison Road, south of Central Avenue and east of Addison Road -- made their debuts yesterday, complete with high school bands, political speeches, balloons, flags and fanfare.

The three openings bring to 40 the number of stations in the system, which is now 37 miles long and 3.52 miles closer to completion. The next section to open will be the Red Line extension up Connecticut Avenue, and that leg isn't scheduled to go into operation until late 1982.

Songs, speeches, smiles and prayers marked the openings of all three stations as the curious, the skeptical, and the eager covered the shiny new platforms like ants attacking candy. The rides were free between the three new stops, and some rode the subterrnaean merry-go-round endlessly because, as one said, "today, it's a bargain for the bored." Passengers will have to pay regular fares starting today.

At the Addison Road station, Lee Ethel Thompson clutched a bouquet of Metro pennants commemorating the opening. She was all smiles and wide-eyed wonder as she prepared to take her first Metro ride, declaring that, come tomorrow, she plans to go "everywhere." Next to her, Berline Norris, who described herself as a Metro aficionado, settled in and told Thompson what to expect.

"It goes so fast your ears might hurt a little at first, but you'll just love it. You'll see. I've ridden them all. I go to all the speeches and collect souvenirs and I think it's just tre-MEN-dous," Norris said. Thompson nodded in agreement, unable to take her eyes off the window. The two women, strangers before the subway ride accidentally brought them together, then began to plot ways to collect more Metro penants and souvenirs.

At the Addison Road ceremony, as Maryland governor Harry Hughes warned the crowd that they might be witnessing the last Metro opening in Prince George's County, and the Central High School marching band played "Ease on Down the Road" from the musical "The Wiz," Andrew Harper, 8 and Robin Wills, 7, were trying to determine how long it would take to ride back and forth between the stations 100 times.

"Look," Andrew said impatiently, "it takes four minutes to go to all three.That's one trip. So it's gonna take up 400 minutes to do all three (100 times)."

"How long is 400 minutes?" Robin asked.

"I don't know, let's figure it out on the train."

Two hours later, they were still at it. Eric Walker, age 2 months, slept peacefully in his father's arms, his tiny pink mouth gently kissing Benjamin Walker's shoulder. Walker and his wife, JoAnne, had come to the Benning station from their home in Capitol Heights to determine if JoAnne could get to her job at the Department of Energy in downtown Washington any more quickly by using Metro instead of driving. They were disappointed, however, when they learned they could not ride all the way downtown during yesterday's ceremonial opening.

"I'm sure they'd want me to use the subway," JoAnne Walker said, "and really, if it's this nice all the time, it would be must better than driving. Why he didn't even wake up," she said, pointing to the motionless little blue-fleece-wrapped body now perched precariously atop her husband's shoulder.

At Capitol Heights, three giggling little girls and one adult with a weary smile boarded the train. The girls all went to sit by the window. The same window. They all wanted to sit by their mother. One of them suggested that if the mother would sit by the window, they could all sit on her lap. Together. Their mother just wants to sit down.

"Every day, since they turned the very first speck of earth, these kids have been asking me when it would be ready. I promised them I'd bring them out on the first day, and this morning they had me up at 7." She looked at her watch and rolled her eyes heavenward. It was 1 p.m.

Unaccompanied children of every size and description crowded the platforms in all three stations. One wanted to know if the train would "hook up with the one in New York City." Another asked an amused police officer if there's a Metro stop at the White House, and began to cry when she learned that she can't go beyond Benning Road today. A third says she will use Metro to go from Benning Road to Nortwest Washington where her boyfriend lives. "But don't write that down or my momma will take away my Farecard." She is 12.