Metro will take another leap forward next month when four new subway stations open under a once-decaying Arlington commercial strip that is beginning to show new life.

The three new miles of tunnel will drive the Orange Line west from Rosslyn through the Wilson Boulevard-Fairfax Drive corridor and will give Metro trains a new destination -- Ballston.

Someday - Metro planners hope as soon as 1985 -- Ballston will be just another stop on the long-planned line to Vienna. But for the interim, the Arlington neighborhood will become an important bus-subway transfer point, and if early signs mean anything, a re-born shopping and residential area as well.

That is a mixed blessing to many in the neighborhood, who complain that the new Metro terminal will also dramatically increase automobile traffic on their streets.

With the Dec. 1 opening of the Ballston line, subway riders on the Orange and Blue lines will have to be a little more knowledgable about which train to take.

That is because trains on the lines will be going to one of four possible destinations instead of one of two. Virginia-bound trains will alternate between Ballston on the Orange Line and National Airport on the Blue line.

Trains crossing from Virginia into Washington will go either to New Carrollton on the Orange Line or only as far as Stadium-Armory on the Blue Line. The two lines share track between Rosslyn and Stadium-Armory. b

Opening the three new miles of subway also means that the metropolitan area's long dream of a 101-mile subway system will have become one-third real. With Ballston, there will be 33.7 miles of operating double-track subway, including 37 stations, all purchased by federal, state and local taxpayers for $2.3 billion, excluding management salaries and subway cars. That works out to a price tag of slightly more than $67 million per mile.

The openings are the only ones scheduled for the subway this year. Officials say the next segment of the systems -- a 3.5 mile stretch of the Blue Line between Stadium-armory and Addison Road in Prince George's County -- won't open until late next year.

The first 30.8 miles have been fabulously successful in terms of ridership and public acceptance. That has probably guaranteed that the system will be completed as planned within the next decade if Congress and the Maryland and Virginia legislatures give area governments the moral support and tax sources needed to finish the job.

The subway is carrying about 260,000 riders every weekday. Metro planners estimate that the Ballston line will add about 10,000 riders each weekday.

There will be a major shift of suburban Virginia bus routes after New Year's that will find many Metrobus riders transferring at the Ballston station on Fairfax Drive at N. Stuart Street, just three blocks from N. Glebe Road.

About 6,500 of those bus riders now transfer to the subway at the Rosslyn station, one of the most crowded stations in the system.

The new line should also benefit the hordes who try to squeeze through the Farragut West Station in downtown Washington during rush hours. Trains will run every three minutes on the shared portions of the Blue-Orange line between Rosslyn and Stadium-Armory. So each train should be somewhat less crowded than those that run every five minutes during rush hour today.

The bad news is that, if a train breaks down, there will be less flexibility for Metro's central control to maneuver trains, thus causing longer delays. People wanting a train to Ballston may also be irritated if two trains go by for National Airport, betfore a train for Ballston.

The four new Arlington Metro stations are almost carbon copies of other underground Metro stations, with high vaulted ceilings and clean lines. The Courthouse station, under the Arlington County Courthouse, has a single platform set in the middle of the two tracks.

The other three stations -- Clarendon, Virginia Square and Ballston -- have two platforms, one on each side. They have an unusually spacious look, because the familiar pylons that stand on all other Metro platforms are missing, and the subway system information that appears on those pylons hangs on the wall instead.

Jackson Graham would hate it.

Graham, Metro's first general manager now retired to Palm Springs, designed Metro's pylons and decreed that graphics that would be set upon them. d The station identifications, he ordered, would be lettered sideways on the pylons so that one would have to crick one's neck to read them. There would be no signs on the walls.

The public complained, Graham retired. The Metro Board decreed that signs would be hung on the walls.

Metro has also added some safety features to the tunnels between stations -- signs indicating how far it is to the closest station or emergency exit and little signs every few feet so train operators immediately can tell central control when they are located in case of trouble. These signs grew out of a bad accident the San Francisco Bay Area Rapid Transit (BART) subway had last winter with a tunnel fire. Similar signs soon will be installed in the existing Metro tunnels.

The Wilson Boulevard corridor over the new Metro line was once the downtown major shopping area for most of Northern Virginia. As the suburbs pushed farther outward, large regional shopping centers opened elsewhere and Wilson Boulevard began to decline.

Recently there have been signs of a recovery as construction workers have closed the ugly gash they carved through the heart of the county as they cut the twin tunnels into the earth.

Major new office building construction has begun around the Courthouse Clarendon station, surrounded by many small parcels of land, is the major business center for Washington area Indochinese refuges.

The Virginia Square station is named for an aging shopping center that once had as its anchor a Kanns department store.

Ballston itself is located two blocks from the Parkington shopping center, where major redevelopment has been proposed to the Arlington County Board. New town house construction is under way in the area.

Henry Hulme, Arlington's County's public works director, said in an interview that the new line "will be such a help to people in the west part of Northern Virginia that a lot more people are going to try and use automobiles to get to Metro. Their problem is there will be little parking. No new lots, by Metro or others, are planned around the station entrances, although some temporary lots may be permitted.

Ballston was never intended to be a metro terminal. Thus the present political battles in Congress and in the state legislatures to secure Metro funding are of particular immportance to Arlington County because more money is needed to extend the Orange Line to Vienna.

Metro is now estimated to cost slightly more than $7 billion. It was supposed to cost $2.5 billion in 1968, before Vietnam War-induced inflation and political foot-dragging on Metro made the estimate meaningless.

Despite the hugh construction cost, however, the largest, long-range concern for Metro planners is the cost of operating the subway system and its vital feeder bus network.

The subway alone is running a $33.7 million deficit in the current fiscal year, since it recovers slightly more than half of its costs from fares. In fiscal 1981, the subway is expected to have a $47.8 million deficit, but a probable fare increase could cut that somewhat.

The Metrobus system, however, is collecting only about 40 percent of its costs from fares and is running a current annual deficit of $94 million. That is projected to reach $109.3 million next year.

Thus, Metro's total bus and subwy operating subsidy in fiscal 1981 is projected at $157 million. It is also expected to continue climbing. The worst-case estimates foresee a subsidy of $500 million to $600 million by 1990. wInflation is assumed in these estimates and the dollar figures are less meaningful than they would appear.

The central issue for area governments is going to be what percentage of their budgets Metro should be allocated. It is currently less than 5 percent, on a regional average.

Sen. Thomas F. Eagleton (D-Mo.), chairman of the Senate's District of Columbia subcommittee, held hearings recently on a pending bill that would authorize $1.7 billion that Metro needs to finish the system.

Eagleton's harshest questions came for local government representatives -- Fairfax County Supervisor Marie Travesky and suburban Maryland Metro Board member Cleatus Barnett. He asked them if local governments had "the will" to meeting growing operations deficit. They said the will was there.

Nonetheless, that question " is the biggest worry I have," Eagleton said in an interview. The Metro system is obviously going to operate in the red. Its deficits are going to range from sizable to horrendously sizable.

"It's one thing for people to say, Oh yes, we have the will. Its another thing for local government to step up and pay the bill when the time comes. I consider that the $64 million unanswered question.