After a year's delay, rail service is tentatively scheduled to start in December between National Airport and Metro's new Huntington station in Fairfax County. The extension includes three intermediate stops in Alexandria.

"It's been sitting here basically complete," said G. Thomas Prince, a Metro construction engineer, as he strolled through the Huntington station. "All the testing has been done."

The transit authority had sought to open the Huntington spur last December, but its plans were thwarted by delays in deliveries of new rail cars. At least 36 additional cars were needed to start service on the extension, according to Metro officials. That goal is expected to be met in the next few months.

Under the transit agency's tentative plans, Huntington and the three Alexandria stations--Braddock Road, King Street and Eisenhower Avenue--would be served temporarily by the Yellow Line. Trains probably will run every six minutes during rush hour and every 12 minutes at other times.

While the opening of the Huntington extension appears only a few months away, plans for a connecting spur to Metro's proposed Franconia-Springfield station in Fairfax County remain clouded by financial uncertainties. The Franconia-Springfield branch is designed to include a stop near Van Dorn Street in Alexandria.

No federal funds have been set aside for the Franconia-Springfield extension. Additional uncertainties have been caused by delays in construction of a controversial proposed Virginia highway known as the Springfield Bypass. The Franconia-Springfield station would not be accessible to commuters, according to transit officials, unless the bypass is built.

Nevertheless, the transit agency has tentatively set 1990 as its goal for opening the Franconia-Springfield route. Some officials have contended, moreover, that the branch could open sooner if federal funds were allocated to start building it.

The four stations included in the Huntington extension bear one further earmark of Metro's changes in plans. Signs posted along their platforms label the stations as part of the Blue Line, as was previously proposed. Before the extension opens, the signs likely will have to be colored yellow.

The still-tentative shift from Blue to Yellow is aimed at allowing the Huntington extension to open by December. Since the Yellow Line is shorter, it would require fewer additional rail cars to handle the Huntington branch, officials have said. If Huntington were included on the Blue-Orange Line, the transit authority might not have enough cars to open it until April.

If Huntington opens as a Yellow Line station, commuters heading for stops on the Blue-Orange Line, such as Rosslyn or Farragut West, would be able to transfer from Yellow to Blue-Orange trains at National Airport and several other stops.

Eventually, however, the colors are expected to be changed again. The Blue Line would operate to Huntington and the Yellow to Franconia-Springfield.

In Alexandria, the rail system's architects have departed from Metro's customary modernism in an attempt to blend the new stations into historic Old Town's atmosphere. The Braddock Road and King Street stations include several traditional flourishes, including decorative lampposts, roofs and brickwork.