Metro opens the first section of its new Yellow Line Saturday, providing Northern Virginia commuters with another link to downtown Washington and marking the end of 10 years of tunnel boring, bridge building and construction work. For thousands of commuters the new line will offer faster trips than they now get on the Blue and Orange Lines.

"It will be a quicker ride, and it will be less expensive," says Fairfax County Supervisor Joseph Alexander, a member of Metro's board of directors. "We hope it will take some of the load off the Blue Line."

Trains will traverse Metro's new Potomac bridge at speeds of up to 55 miles an hour, according to Metro officials. From their windows, rush-hour subway passengers will have a view of slower-moving automobiles and occasional traffic backups on the 14th Street bridge, Washington's most heavily used crossing.

A Blue Line trip between the Pentagon and L'Enfant Plaza stations now takes 15 minutes, official say. The new 3 1/4 mile section of the Yellow Line will provide a six-minute ride between the same two stops.

The Yellow Line's opening, nevertheless, has led to complaints from some Northern Virginia bus riders. Two months after the line opens, Metro plans to end most bus service across the 14th Street bridge--a move expected to cause inconvenience and delays of 5 to 15 minutes for nearly 8,000 commuters. They will be forced to get off their buses at the Pentagon and transfer to subway trains or special shuttle buses.

In addition, rush-hour trains on the Red, Blue and Orange Lines have become more crowded since Metro took some rail cars from these older lines and put them on the Yellow Line.

Trains on the Yellow Line are scheduled to leave National Airport every six minutes during rush hours, making stops at the Crystal City, Pentagon City and Pentagon stations. Then they will cross the Potomac, tunnel beneath the Washington Channel and stop at L'Enfant Plaza, where riders may transfer to the Blue Line.

Yellow Line trains will also stop at a new Archives station at Seventh Street and Pennsylvania Avenue NW near the National Archives, and will terminate at Gallery Place, a transfer point for the Red Line.

A new entrance to the Gallery Place station will open at Seventh and H Streets NW in the heart of the city's Chinatown area.

Its sign has been embellished with Chinese characters, which Asian community leaders say may be interpreted in two ways. They may be loosely translated as "The Beautiful Community Subway Station," while phonetically, the characters sound like the English words, "Gallery Station." Community leaders express hope that the new line will bring patrons to Chinese restaurants and shops and spur development in Chinatown.

Subway passengers who now ride the Blue Line between Northern Virginia and the District will face a fundamental choice: Should they continue using the Blue or switch to the Yellow?

Metro officials have suggested a rule of thumb: A key factor is whether a passenger from Northern Virginia now rides the Blue Line to a stop east or west of Metro Center. For trips between National Airport, Crystal City, Pentagon City or the Pentagon and any Blue Line stop in the District east of Metro Center, taking the Yellow Line will save time, officials say. Otherwise, the Blue Line will be faster.

For example, a Blue Line ride from National Airport to the Capitol South station now takes 27 minutes. By choosing the Yellow Line and transferring to the Blue at L'Enfant Plaza, rush-hour commuters can make the same trip in about 17 1/2 minutes, officials say. The faster ride would include 12 minutes on the Yellow, 4 minutes on the Blue and an average of 1 1/2 minutes spent in transferring.

A trip from the Pentagon to Metro Center would point to a different conclusion. On the Blue Line, the ride takes 12 minutes. If a passenger starts out on the Yellow and transfers to the Blue, the trip will take about 12 1/2 minutes, including 1 1/2 minutes for the transfer. Metro officials say such passengers should stick to the Blue.

As preparations for the Yellow Line's opening entered their final week, Metro officials carried out last-minute tests, installed signs and cleaned up debris left from construction work. An elaborate fire drill was conducted Friday by police, firefighters and other emergency workers on a train midway across Metro's Potomac bridge.

"My 300-page list is down to about a page and a half," said Alan Peck, a Metro construction engineer, a few days ago as he strolled through the new Gallery Place entrance, where panels were still missing from escalators and fare machines were cloaked in plastic casings.

Little hint remained of the clanging, pounding, rumbling and dust that once filled the Yellow Line's cavernous stations and tunnels. The mammoth project, which began on Feb. 12, 1973, when workers started excavating the Archives station, cost millions of dollars and disrupted downtown streets for years.

Twin 4,000-foot tunnels were carved out in the mid-1970s to allow trains to run underground from L'Enfant Plaza to Gallery Place. The tunneling was done mainly by a huge steel contraption shaped like a cookie cutter and shoved along by hydraulic jacks.

Enormous steel tubes were towed behind tugboats up the Potomac River from a manufacturing plant northeast of Baltimore to provide a tunnel for the Yellow Line beneath the Washington Channel. "They plugged up everything, filled them with compressed air and they just floated," recalled Edward L. Waddell, Metro's construction director.

In all, the first section of the Yellow Line cost $186 million to build, including the $18.5 million Potomac bridge--far less than a similar project would cost today when, Waddell noted, a single station might go for $90 million. The Yellow Line was once scheduled to open in 1977, but it was delayed because of changing transit plans.

The line's future is marked by uncertainties. As an initial step, on June 28 Metro officials plan to eliminate most bus service across the 14th Street bridge in an attempt to save $1.7 million. Officials say the old bus routes would duplicate the new rail service needlessly. At the same time, a new Metro shuttle bus, to be called the Route 13, will start running between the Pentagon and downtown Washington via the 14th Street and Memorial bridges. These buses will provide service to areas not easily reached by the subway.

In December, Metro officials plan tentatively to extend the Yellow Line to a new Huntington station in Fairfax County south of Alexandria.

Huntington was planned as a stop on the Blue Line, not the Yellow. However, a shortage of rail cars has prompted Metro officials to propose the temporary switch. Since the Yellow Line is shorter than the Blue, it will require fewer trains to operate. Metro officials say they probably can get by with fewer rail cars if they extend the Yellow Line, rather than the Blue, to the Huntington station.

In 1990, the southern end of the Yellow Line is scheduled to be extended to a Van Dorn Street stop in Alexandria and a Franconia-Springfield station in Fairfax County.

A year earlier, according to current plans, the northern end of the Yellow Line would be extended to offer service to stations at Mount Vernon Square, Shaw and U Street NW. These stops are also included on the Green Line, a route presently embroiled in neighborhood controversies and court conflict. The Green Line is planned to run from southern Prince George's County through Anacostia to Greenbelt, also in Prince George's. If the Green Line is completed, officials say, Yellow Line service would end at Mount Vernon Square, where passengers could transfer to the Green.