Both sides may have won a victory in the dispute over whether a rail transit line should be built in the median strip of Interstate Rte. 66 in Virginia - if the controversial I-66 itself is extended inside the Capital Beltway.

An official regional committee agreed last week to consider whether the rail line should be:

Built, as originally planned, in the I-66 median from Arlington to a terminal at Vienna, in Fairfax County.

Built in the I-66 median only as far as West Falls Church station, then diverted northwestward to a terminal in the Westgate-Tysons Corner area.

Truncated at the Ballston station at Glebe Road in Arlington.

The route already is under construction in a subway from Rosslyn to Ballston where, under current plans, it would reach the surface and follow the I-66 median.

The decision was reached by a joint steering committee formed by the Metropolitan Washington Council of Governments, the COG transportation planning board and the Metro board to oversee a study of possible cutbacks and alternatives to parts of the projected 100-mile Metrorail system.

There was no clear-cut winner in the dispute, which - in general - pits those who want the entire line built against some who believe current plans for building I-66 as a commuter busway make the rail line unnecessary. Chairman John F. Herrity of the Fairfax County Board of Supervisors is a leader of the Metrorail doubters.

The dispute has a contradictory backdrop. Former U.S. Secretary of Transportation William T. Coleman Jr. and Brock Adams, who now holds the office, approved the I-66 project because it was one way of getting the Metrorail line built too.

Although the committee decision merely set in motion a consultant's study that will take at least until midyear and then will be submitted to political officials for a decision, it raised the first official possibility that - even with I-66 approved - the rail line may never be built beyond Glebe Road.

Some committee members who supported making the study, including Fairfax County supervisors Joseph Alexander and John P. Shacochis, however, predicted it woull erase any doubt that the line is necessary.

Joseph S. Wholey, chairman of the Arlington County Board and first vice chairman of the Metro board, accepted that as a "strong argument." Wholey, who had insisted that no consideration be given to a permanent terminal at Ballston, dropped his opposition to the broad study.

Only Mayor Harold F. Miler of Falls Church voted against making a study under which West Falls Church would be considered as a terminal for the route.

Hours after the COG-Metro meeting, the Falls Church City Council took what might become a decisive position in the matter.

It voted to put strigs on Metro's planned acquisition of a six-acre tract of city-owned land needed for a vital rail-car-storage yard at West Falls Church. If Metro does not build the line to some point farther west, the council decreed, Metro cannot used the land for its yard.

The council fears that the Western part of the city and the city's George Mason High School, located near the site of the West Falls Church station, would be flooded by traffic from buses and automobiles if the rail line were to be terminated there.

In taking its action, the COG-Metro committee decided to drop any consideration of using "light rail" trolley cars to provide service at the outer end of the Metrorail line.

On another Virginia rail route dealing with the line that is planned through Alexandria, the COG-Metro committee decided to study the outright abandonment of the proposed route from Alexandria to Springfield-Franconia and the possibility of terminating the route at the Van Dorn station in south-western Alexandria.