The Metro system's plans for extending rail service to Alexandria and Fairfax County next December have been thrown into doubt as a result of recent delays in testing new rail cars, reliable sources said yesterday.

In the past, Metro officials have said they needed 36 new, fully tested rail cars before they could open the four-mile extension between National Airport and Fairfax County's Huntington station. "We're not going to have 36 cars by the end of December," a senior Metro official said yesterday.

Because of the delays in testing the Italian-made cars, Metro officials may have to consider postponing the Huntington extension's opening, officials said. Such a move would probably stir controversy, partly because the Huntington spur's opening has already been set back by a year.

Another option, officials said, would be to maintain the current December schedule for opening the Huntington branch by shifting to less frequent service and shorter, possibly more crowded trains. Officials said they do not expect to decide what steps to take for at least two weeks.

The transit authority's plans for expanding its rail network have repeatedly been disrupted because of delays in rail car deliveries, controversies and shortages of funds.

Shipments of rail cars previously were stalled by a seven-month labor union strike at a Pittsburgh brake factory and a breakdown in essential manufacturing equipment at a West German plant.

Metro officials, who asked not to be identified, attributed the recent delays to the way tests of the newly assembled cars have been carried out by their manufacturer, Breda Costruzioni Ferroviarie of Pistoia, Italy.

The tests, conducted by Breda employes at a Metro rail yard in Alexandria and an Amtrak facility in Beech Grove, Ind., are designed to ensure that the cars operate reliably and safely.

By now, 12 cars had been scheduled to have been delivered to the Alexandria yard, but only two have arrived, officials said. In addition, key tests performed on the first two cars were expected to have been completed by now. Instead, officials said, these tests appear unlikely to be finished until mid-September. Moreover, the tests have already uncovered a troublesome defect in the cars' brake systems, officials said.

Officials could not be reached for comment yesterday at Breda's headquarters in Pistoia. An official at Breda Transportation, Inc., a New York-based subsidiary of the Italian firm, said he was not prepared to comment on the issue.

The Huntington extension is designed to include three stops in Alexandria--at Braddock Road, King Street and Eisenhower Avenue. While plans for Huntington appear in jeopardy, Metro officials said, the delays in rail car tests are unlikely to disrupt schedules for opening other branches, such as the planned extension to Shady Grove in Montgomery County.

Fairfax County Supervisor Joseph Alexander, a member of Metro's board of directors who has pressed for the Huntington station's opening, said yesterday it would be premature to suggest that service might be delayed. "In two weeks, we'll have to decide what's going on," he said.

Arlington County Board Vice Chairman John G. Milliken expressed concern about any attempt to open the Huntington branch with too few cars. He warned that such a move might put a "squeeze" on other parts of the rail system and might cause further overcrowding.

In a related development, the transit authority made public a proposal for cutbacks and other changes in 46 bus routes in Northern Virginia. The plan is designed to revise bus service to tie in with the Huntington rail extension. Because of objections to the cuts by Alexander, the agency temporarily put off plans for a public hearing on the proposal.