Along the I-66 median, construction crews are erecting Metro stations and preparing to lay tracks for a nine-mile extension through Northern Virginia scheduled to open in 1986.

The extension is designed to connect the Ballston station in Arlington County, now a terminus of the Blue and Orange lines, with a new Vienna stop in Fairfax County. It would include intermediate stations at East and West Falls Church and Dunn Loring.

The transit authority has tentatively planned to start service on the Vienna branch in mid-1986, but Metro officials recently warned that a Senate bill designed to limit outlays for subway construction may lead to a six-month delay in the extension's opening.

Even before rail service begins, some transit officials are seeking to make use of two parking lots scheduled to be built near the stations. The Northern Virginia Transportation Commission has proposed setting up a special shuttle bus system to carry commuters from the lots at West Falls Church and Vienna to the Ballston station.

Today those lots appear to be little more than muddy fields. The Vienna station has just started to go up. Miles of steel rails are stacked at a yard, ready to be laid. New roads leading to the stations must be built. Walkways crossing I-66 must be completed. After construction has ended, elaborate tests of electronic equipment remain to be carried out.

Nonetheless, work has moved rapidly, especially at the East and West Falls Church stations. Tiles are already being laid on the platforms. Skylights have been installed.

Metro officials point to the earthen hues of the new stations, noting the way they appear to blend with the subdued coloring evident on lampposts, signs and other structures along the recently opened section of I-66 east of the Capital Beltway.

"The stations are just about finished, but we still have all the track work to put in and the automatic train control to put in and the communications," said William S. Pauling, a Metro construction engineer, as he stood outside the West Falls Church station. "We're really running tight to be able to open in July of 1986."