William T. Scoggin had no idea when he left home in Centreville, Va., yesterday morning that by afternoon he would be the spokesman for thousands of Metro employes, like himself, whose strike has brought Washington's mass transit system to a painful, grinding halt.

Scoggin never had taken much of an active interest in his union's activities, his wife, Elizabeth, told a reporter yesterday. On Tuesday night, while many union members were caught up in a tumultuous meeting with the leaders they have come to distrust intensely over the years, Scoggin was driving his bus.

On Wednesday night, he told his wife he was afraid there would be a strike. "But I'm going in tomorrow," she remembered him saying, "and if there's a picket line I'm not going to cross it."

After the first day, however, it became apparent to Scoggin, his fellow drivers at the Arlington garage and to many other strikers around the city that the job action they had begun could not possibly end in success unless they could speak with a united voice.

The men in Arlington were the first to act. They persuaded a radio reporter covering the strike to broadcast an announcement, calling together representatives of all the strikers in one of the vast parking lots near RFK stadium.

By the time Scoggin and his friends arrived there at 9:30 a.m., yesterday, almost 100 people stood waiting in the burning sun or seeking shade beneath the now quiet Metrorail bridge that spans the lot.

Scoggin and another Arlington driver, Bill Ottinger, introduced themselves. Then Scoggin began to speak.

"Our whole aim is to get everybody back to work as soon as possible, without any of us getting hurt, he told the group. "As long as we're not organized, just sitting around the garage having nice little . . . conversations . . . we're not going to get anywhere."

Soon an organization began to take shape as groups from each garage caucused and selected a final assembly of about 20 people who retired beneath the shade of a tree to try to determine the future of the strike.

"We're here right now," Scoggin told reporters who thrust microphones in his face, "because previously there was no leadership. The union has left us out a long time ago. They never had control of the rank-and-file . . . We're all aware that the union is promanagement and has not had our interests in mind for a long time. We are the union" The crowd erupted into cheers.

"This is a revolution of sorts," said a driver from Bladensburg with a broad smile - and the revolution had found in Scoggin, if not its leader (even he insists there is none), then its voice.

The decision may have surprised Scoggin himself but not his friends.

Willis Shinaberry, another Arlington driver, who has known Scoggin since they trained together 11 years ago, described him as a no-nonsense person.

"If somebody comes up to talk about fishing, he's interested but he won't carry on a long conversation. But if you've got a problem - like if you tell him another man and you want to kill him - he'll tell you 'Don't do that,' and there's legal representation for you."

Yet few of the other drivers in Arlington knew the 44-year-old Scoggin well enough to give details about his background. When he was not busy driving - his wife said he worked as much overtime as possible - then he was with his family, his two teen-aged boys or his grown daughters. Or he was reading.

"Between books and children," Elizabeth Scoggin said, "that's what we have. He reads, oh, everything. What's the favorite? Churchill, I guess."

Scoggin told people today that he was from New Mexico, but he rarely mentions that his father was a state district judge there, that he had almost four years of college - including law courses - at the University of New Mexico or that he spent 17 1/2 years as a policeman in New Mexico and on Capitol Hill.

When he moved back to Washington 11 years ago, his wife said, Scoggin chose bus driving over police work because the pay was better. "He almost didn't get hired by the bus company, she said with a laugh, "because they said he was overqualified and wouldn't stay."