Metrorail grows again Sunday morning, this time gaining three new stations and 2.5 miles of track in Northwest.. Shortly after 10 a.m., the first train with paying passengers will travel through the twin, deep-rock tunnels that extend north from Dupont Circle to Van Ness-UDC.

The weekend also will give Metro riders cause to complain: Higher subway fares go into effect tomorrow, raising base fares to 65 cents and increasing rush-hour mileage fees. Most base bus fares will go up to 65 cents on Sunday, with zone crossing fees rising as well.

The new section will put just over 39 miles of track in operation throughout the area -- not yet half of the 101 miles shown in the long-delayed subway system's master plan. To mark the occasion, the first extension since the Blue Line opened to Addison Road a year ago, Metro has organized a day of civic celebration on Saturday.

From 8 a.m. through 4 p.m., the three new stations -- Woodley Park-Zoo, Cleveland Park and Van Ness-UDC -- will be open. Passengers boarding at the new stations Saturday will be able to ride free anywhere in the system by picking up passes at the stations, each good for a single ride before 4 p.m.

There will be separate opening ceremonies at each of the three stations, beginning at 10 a.m. at Woodley Park-Zoo station at Connecticut Ave. and Woodley Road.

D.C. Mayor Marion Barry and other civic dignitaries will be on hand with speeches; the traditional ribbon will be cut by a person in a panda suit. The Sheraton Washington Hotel will provide hot cider and coffee. The zoo will host a four-hour festival running until 2 p.m., featuring live music, films and a London double-decker bus.

At 11 a.m. ceremonies begin at Cleveland Park station at Connecticut and Ordway St. School choruses will perform and there will be light refreshments at the local library, a transportation exhibit and a display of winning poems and posters from a contest for children.

One hour later, Van Ness-UDC station at Connecticut and Van Ness Ave. , the last stop until the Red Line is extended further north (now scheduled for late 1983), will be formally dedicated. Merchants from the Van Ness Centre will organize a celebration with food, a dance troup and a drawing for prizes.

Metro officials estimate that the three new stations will generate about 18,000 new rides a day after new bus schedules go into effect Jan. 31.

The twin tunnels deep below Connecticut Avenue are 17 feet in diameter and 36 feet apart. They were bored and blasted through rock in work that began a decade ago. In the space of a mile, they will provide passengers with some of the deepest -- and shallowest -- rides in the Metrorail system.

There are only five or six feet between the tunnels' reinforced ceilings and the bed of Rock Creek below the Connecticut Avenue Bridge. Moving uptown from there, the passageways climb gently toward the high ground of Upper Northwest, leveling out for station platforms. Their deepest point, about 160 feet from track to street, is located just below the entrance to the Zoo.

Tunneling between Dupont Circle and Rock Creek was finished by March 1973. The next month, mining teams began a contract to extend the tunnels from the creek northward, finishing the job in October 1976. Station construction got under way in 1976, with work on track, automatic train control wiring, Farecard equipment and other "staging" items starting in February 1979. The total cost came to $181 million, Metro says.

There were a few hitches, though not particularly serious ones, in the long years of contruction. Several times, rock structures crumbled below ground, one of them collapsing a wooden street "deck" near the Cleveland Park station.

Water seeped into Woodley Park-Zoo in unforeseen volumes, forcing workers to pack a sort of calking into rock cracks. Engineers still don't know why it's so wet at that spot.

Commuters entering the new stations for the first time will be surprised to find a new, utilitarian archicture overhead. Those waffle indentations familiar in the downtown vaults are missing. The concrete panels are large and simple, fitting together in precise units of four to put a roof over the platforms.

It is an economy measure. The panels were precast in Winchester, then hauled underground and erected. About 280 panels make up each station's ceiling and sides. System-wide, this will save about $10 million over casting in place, as was done in the earlier stations, Metro says.

Despite their bulk and appearance of weight, the vaults hold nothing up other than themselves. There is a good six feet of empty space between the vaults and the roofs of the rock caverns that house the stations. Those massive arches are for the sake of appearance and control of water and air.