Now that the federal government has cleared the way for extending Interstate Rte. 66 inside the Capital Beltway, the fate of the Metrorail transit line planned to occupy its median strip has become uncertain.

John F. Herrity, chairman of the Fairfax County Board of Supervisors, wants to abandon the rail line entirely, contending that the use of I-66 as a busway would make it unnecessary.

Fairfax County Supervisor John P. Shacochis, who represents the McLean area, who represents the McLean area, wants to reroute the rail line from its proposed Vienna terminal in the I-66 median to a new alignment serving Westgate, a major employment center near Tysons Corner.

Former U.S. Secretary of Transportation William T. Coleman Jr., in approving the I-66 project before leaving office in January said Virginia's promised financial support for building Metro in the median strip was a factor in his decision. His successor, Brock Adams, voiced similar views in reaffirming Coleman's decision.

Under the plan, I-66 from the Capital Beltway to Rosslyn would be only four lanes wides and restricted to buses and car pools during rush hours.

While the I-66 approval binds Virginia's state government to supporting Metro, it does not actually require Metro to built the line.

Ironically, the victory recently won by Fairfax City in its lawsuit against Metro could make it possible to achieve either the abandonment or the rerouting of the rail line. Fairfax City was seeking to force construction of the line to Vienna, the station that would serve the city, or to get back the $2 million it has contributed toward the project.

Under th terms of a ruling made Feb. 25 by U.S. District Judge Oren R. Lewis, Metro could pay back the $2 million and would then be freed to change its mind about the route.

The new routing possiblities came to light last week at a meeting of a joint steering committee that it overseeing a study of possible cutbacks or other changes of the 100-mile regional Metrorail system.

The steering committee represents the Metropolitan Washington Council of Governments, the COG transportation planning board and the Metro board.

One of the routes being studied for possible alterations is the Vienna route west of Glebe Road in Arlington. The line already is being built from Rosslyn as far as Glebe Road.

(The Glebe Road station was offcially renamed the Ballston station by the Metro board last Thursday. The next station toward downtown from Glebe Road, formerly called Ballston, will now be called Virginia Square, named for the adjacent shopping center.)

Herrity, a persistent critic of Metrorail construction into Fairfax County, suggested at the steering committee meeting that the federal approval of I-66 as a commuter arterial makes the rail line unnecessary.

Herrity said I-66 as a busway, similar to that in I-95 (Shirley Highway), would be "a mass transportation facility" that would be "essentially competing . . . with another mass transit facility (Metrorail) down the middle."

His statement drew a rebuke from Joseph S. Wholey, chairman of the Arlington County Board and first vice chairman of the Metro board.

If Herrity reflects Fairfax County's official position, Wholey said, "Fairfax County should think again." Wholey said he supported Herrity in seeking a restudy of the Metro extension to Vienna "because I though there were alternate routings . . . but I didn't think one of those (possibilities) is not to build it at all."

Shacochis, who serves on both the Fairfax County Board of Supervisors and the Metro board, told Wholey: "I would like to say that Mr. Herrity speaks for himself," not for the entire Board of Supervisors.

Shacochis said he will propose to the steering committee next week that a rerouting of the line to Westgate be considered.

Under that plan, the route would be built on its already approved alignment in the I-66 median from Ballston (Glebe Road) to West Falls Church, then would tilt northwestward toward Westgate and, eventually, would be built to Dulles Airport in the median of the airport access highway.

Herrity told a reporter this would be prohibitively expensive.

Harold L. Miller, mayor of Falls Church, said his city is determined that the rail line, if built, should extend to a terminal some distance west of Falls Church. He said any possibility of terminating the line at the West Falls Church station would be intolerable because of the traffic impact.