With as much showmanship as could be mustered in a maintenance garage, Metro officials unveiled two new Italian-made subway cars yesterday, touching off renewed predictions that rail service will reach Fairfax County by December.

"They're beautiful," said Richard S. Page, Metro's general manager, as he stood beside the shiny 75-foot cars now undergoing exhaustive tests at the transit authority's rail yard in Alexandria. Page was accompanied by Fairfax Supervisor Joseph Alexander and Alexandria Mayor Charles E. Beatley Jr., who hailed the cars as a propitious sign for Virginia commuters.

The rail cars, the first delivered to Metro in five years, are essential to the transit authority's plans to extend Yellow Line service to Metro's Huntington station in Fairfax by the end of this year. The line includes intermediate stops at Braddock Road, King Street and Eisenhower Avenue in Alexandria.

Metro needs 36 new cars, officials said, to allow the Huntington extension to open. With the recent arrival of the test cars, officials added, their goal appears within reach. Only a further delay by the manufacturers or an as yet undetected flaw in the freshly built cars could throw their plans off course.

Mishaps have already slowed delivery of the cars, which were once expected to arrive by 1981 or early 1982. Protracted delays stemmed from a breakdown of manufacturing equipment at a West German plant that produced essential panels and from a strike at a Pittsburgh brake factory.

If the new cars had arrived on time, the rail line to Huntington would already be open, Metro officials said yesterday. The line has been built and undergoing testing for several months. Metro officials say the cars are passing their tests, and that the manufacturer is now on a schedule that would provide about 50 new cars by December.

Yesterday, Fady P. Bassily, Metro's chief vehicle engineer, strolled through the two new cars, outlining some of the 60 rigorous electronic and mechanical tests they must pass before being put into service. A number of tests have already been administered, and Bassily noted, "They have satisfactorily performed."

To a casual glance, the new cars resemble those in Metro's existing fleet, with cushioned seats, carpeted floors and a mixture of subdued colors and sleek shape. Nonetheless, officials said, nearly 40 significant changes have been made to make the cars safer, less likely to break down and easier to ride. Some of the shifts proved controversial.

In one such move, Metro's board of directors decided to provide more standing room in the cars by reducing the number of seats from 80 to 68. Seats were eliminated near the front and rear doors. Advocates of this plan argued that the trains could hold more rush-hour passengers-as many as 252 jammed into each car. Metro's current cars hold a maximum of 240 riders, officials said. Opponents complained that the change reduced comfort.

New electronic destination signs have been installed in an attempt to reduce confusion among passengers. Metro's present signs frequently break down, leaving patrons uncertain about where trains are heading. The new signs are said to be less likely to malfunction.

The cars are equipped with additional hand railings to make it easier for standees to hang on. Railings affixed to seat backs have been padded with rubberlike grips, which are described as safer and softer to hold. New speckled carpeting was chosen to "hide the dirt a little bit," as Bassily put it.

Among changes made to reduce breakdowns or enhance safety were installation of extra fire extinguishers, additional air-conditioning equipment, improved braking systems and more resistant floor panels. Equipment in the operator's cab was also slightly altered because of safety officials' recommendations.

The new cars will supplement Metro's 298-car fleet. The current cars were produced in the 1970s by Rohr Industries, which no longer makes transit vehicles. The new cars are being manufactured by Breda Costruzioni Ferroviarie, an Italian train producer. Metro plans to buy 294 Breda cars for about $275 million.