It took 10 years, and there were long periods of doubt, but the Metro Orange Line finally opened ceremonially yesterday in far Northeast Washington and Prince George's County.

Despite overcast skies and intermittent rain, hundreds attended three ribbon-cutting ceremonies and thousands took free rides back and forth on trains that will begin carrying farepaying passengers into downtown Washington on Monday.

"I have to admit there were a lot of times I thought it would never get done," Winfield M. Kelly Jr., the outgoing Prince George's County executive, told spectators at a ceremony at the line's New Carrollton terminal. "We've come a long way, baby," he concluded.

The new 7.6-mile line, an extension of Metro's Blue Line northeastward from the Stadium-Armory station, is the first to reach from the city to the Capital Beltway. It will serve predominantly black neighborhoods of Northeast Washington and predominantly white suburban areas, warehouse and industrial concentrations, in Prince George's.

Theodore C. Lutz, Metro's general manager, told students who attended a ceremony at Woodson Junior High School near the Minnesota Avenue station that the combination of Metro and the suburban commercial activity will be important to them.

"We hope Metro makes [job] opportunities more available to you," he said.

Although serious consideration of a rapid transit rail system for the region dates back to 1959, the crucial political decision was made 10 years ago this month when voters in Prince George's and in suburban Virginia approved bond issues to provide local shares of the needed funds.

Governing bodies of other political units in the region took similar actions. The Orange Line to New Carrollton was one of the first routes to be agreed upon.

But it wasn't always easy, Metro planner William T. Fauntroy Jr. recalled yesterday while standing on the Minnesota Avenue platform crowded with smiling, cheering youngsters waiting for a train to New Carrollton.

Residents of Northeast, long alienated from the rest of the city, have voiced doubts at public hearings in past years about the benefits Metro might bring. "It was great today to hear those great and generous speeches by those who once gave us the most hell," Fauntroy said.

D.C. City Council member Willie J. Hardy, who represents the neighborhood, grinned when she heard envy expressed by John P. Scheochis, a Fairfax County supervisor whose area will not get train service for at least four years.

"You know," Hardy responded. "I think this is the first time I've ever heard anybody from the state of Virginia say to us, 'You've got what we want." It's usually the other way around, she observed.

Kelly got to the Minnesota Avenue ceremony on time by riding a special Metro train, But D.C. Mayor Walter E. Washington was late his chauffeur-driven limousine got stuck in a traffic jam.

Washington defeated this year for reelection after 10 years in office, got the day's only standing ovations at both the Minnesota and New Carrollton cermonies. "How beautiful you are," he told the audience, obviously moved by the tribute.

Referring approvingly to the Metro partnership between the mostly black city and the mostly white suburbans, Washington said: "We used to say we came over on different ships, but with Metro we're now all in the same boat."

A marching band display at Woodson was washed out by the rain, but the New Carrollton ceremony - held in the station mezzanine into which 500 people had crowded - was serenaded by the High Point High School band, playing "Jive Talk" and then "I've Been Working on the Railroad."

Sen. Paul S. Sarbanes (D-Md.) cluching a Metro pennant, said he was keeping pressure on U.S. Secretary of Transportation Brock Adams for federal financial support to complete all 100 miles of Metro.

Adams, speaking next and beaming broadly, said "the federal government is with you, and is going to be with you, and the money is going to be there to do it." But he said the region should "put in some kind of regional tax and get your act together" for paying Metro's operating costs and retiring its bonds.

Yesterday's third ceremony was held at the Cheverly station, also in Prince George's.

Regular service on the Orange Line will begin at 6 a.m. Monday. There will be no service on the line over the weekend.