Northern Virginia politicians and officials tentatively reduced yesterday the number of possible Metro subway alignments in Virginia that could result from the regionwide restudy of the rail system.

In an attempt to develop a unified position before the full regional task force meets again Wednesday, the Virginians tentatively:

Eliminated West Falls Church from further consideration as the end of the Orange Line, which is presently planned to run from Rosslyn to Vienna.

Agreed to consider rerouting the Orange Line at West Falls Church from Vienna to Tysons Corner.

Agreed to study an extension of the Yellow Line from downtown Alexandria's King Street station to two possible terminals, Van Dorn Street in western Alexandria, and Franconia, near the south-east corner of Springfield Mall in Fairfax County.

The remaining possibilities leave many knotty problems unsolved, but the action reflects a major effort by Northern Virginia politicians to reduce their differences. They were encouraged to do Saturday by Gov. John Dalton. Dalton transferred $38 million in highway money to Metro, a first for a Virginia governor.

But problems remain with yesterday's tentative plan. For one thing, Alexandria's political leadership was not represented in the early morning meeting yesterday because Council member Robert Calhoun had the flu. There are significant differences between Alexandria and Fairfax County on the Franconia route - which Fairfax wants to study and Alexandria thinks will cost too much to operate.

Fairfax County Supervisors Joseph Alexander and Marie Travesky will appear at the Alexandria City Council meeting tonight to explain their position and seek Alexandria's support.

There is substantial support on the Alexandria Council (and in the federal government as well) for ending Metro at Huntington. That would mean never running the Yellow Line west to Van Dorn Street and Franconia.

The major concern of the local jurisdictions is not the cost of construction, but of operation. Studies done by Peak Marwick Mitchell and Co. (PMM) for the regional task force have shown that every mile constructed means higher operating costs, regardless of ridership.

The line from King Street in Alexandria to Van Dorn would be about four miles long - all of it on the surface. There would be no intermediate stations.

The Van Dorn station close to an exit from the Beltway would be available not only to the residents of Alexandria's west-end high rises but also to may Fairfax County residents. But all the track to get to Van Dorn and the station itself would be within the Alexandria city limits.

That would mean under the present regional cost-sharing formula that Alexandria would have most of the operating cost while Fairfax County provided many of the riders.

Alexandria is on record as considering Van Dorn only if it can negotiate a cost-sharing agreement with Fairfax.

So far Alexandria has rejected any extension beyond Van Dorn - to Franconia for example - as not cost effective. The PMM studies show that 75 percent of the riders who would get on the line if it goes as far as Franconia would also get on if it terminates at Van Dorn.

County politicans such as Alexander feel that the ridership for Franconia is underestimated and that foes of the extension fail to take into account the completion of a Springfield by-pass highway connector near the station. "I am sure the county can work out a general agreement with the city" on operating costs Alexander said yesterday.

Another objection to any version of the Franconia line has been that the area is already well served by high-quality express bus service on Shirley Highway. However the PMM studies show a need for many more buses to provide service as good as Metro's. Furthermore if the Shirley Highway buses continue to unload at the Pentagon Metro station as they do today the station will have to be vastly expanded. Fare gates and escalators already are badly overloaded during the rush hours.

The issues are just as difficult on the Orange, or Vienna, line. PMM studies show that Metro system ridership probably would be higher if the line were rerouted at West Falls Church to Tysons Corner. Furthermore, such a routing would make it easy in the future to extend the rail from Tysons to Dulles Airport along the Dulles Access Road.

The big problems, however, are time and money. The line to Vienna is planned and ready to go. Environmental impact statements have been completed. Metro estimates it would add four to five years to the 1981 completion date and $60 million in costs if the route were changed to Tysons Corner. Inflation would drive up the costs and the requirements for a new environmental impact statement, public hearings, and plans would increase the time.

Those factors weigh heavily on Arlington and Falls Church. The Orange Line is under construction to the Ballston station in western Arlington County near N. Glebe Road and Fairfax Drive. That segment is scheduled to open from Rosslyn in 1979.

Neither Arlington nor nearby Falls Church is anxious to have Ballston be the end of the line - and attract all that traffic - to inner suburban business neighborhood for any longer than necessary.

"Are you sure it will take another four or five years?" Arlington County Board member Joseph Wholey keeps asking Metro officials. He asked the question again last Thursday at a Metro Board meeting, which was Wholey's first as chairman.