The number of passengers riding Yellow Line trains has fallen markedly short of the Metro system's forecasts, the transit authority said in a report issued yesterday.

The Yellow Line, which opened April 30, is used by 30,000 riders on weekdays--about 20 percent less than the 38,000 passengers previously predicted by Metro officials. Despite recent gains in ridership, transit officials said they did not expect their forecast totals to be reached.

The dearth of riders has been especially pronounced during rush hours. Only 3,100 commuters take the Yellow Line from Northern Virginia to the District during the busiest morning hour on weekdays, the authority said. The number is 38 percent less than the 5,000 passengers predicted for that hour.

The limited number of Yellow Line riders has caused concern among transit officials, who are seeking to hold down the rail system's costs. One preliminary suggestion for reducing costs, made public yesterday by Metro officials, would eliminate all Yellow Line service on weekends and weekday evenings, when ridership is lowest.

At the same time, however, officials said the paucity of patrons may make it easier for Metro to extend Yellow Line service to Alexandria and Fairfax County in the next few months.

The Yellow Line, which now runs between National Airport and Gallery Place, is tentatively scheduled to be expanded in December to serve three stations in Alexandria and a Fairfax County stop near Huntington Avenue. Metro's plans, however, have recently been thrown in doubt because of unexpected delays in testing new rail cars, which are needed for the Huntington spur.

If too few cars are available, officials have said, one way of avoiding a postponement of the Huntington branch's opening might be to shift to shorter trains or less frequent service. Since Yellow Line trains now are only about half-filled at rush hours, officials noted, the rail system may be able to handle more Alexandria and Fairfax commuters without as many new cars as had been planned.

Metro officials attributed the shortfall in Yellow Line ridership partly to reluctance among bus riders to switch to the subway system. Officials also expected many commuters to start taking the Yellow Line after most bus service over the 14th Street bridges between Northern Virginia and the District was halted on May 26.

Some bus riders, however, transferred instead to a special shuttle bus that started running between the Pentagon and the downtown area. Other bus patrons "abandoned transit altogether," the Metro report said. The cutbacks in bus service caused widespread complaints among commuters.

Another factor, Metro officials said, was that fewer rail passengers switched to the Yellow Line from the Blue and Orange Lines than had been predicted. Officials attributed this pattern partly to lack of familiarity with the Yellow Line and partly to reluctance among some passengers to transfer between the Yellow and the Blue or Orange Lines.

In addition, the report said, "It does not appear as if all passengers who would be better served by the Yellow Line are being selective about the train they take. In the case of inbound travel in the morning from the Pentagon, many passengers seem to take the first train that comes into the station, regardless of their destination."

Metro officials also said they may have erred in their forecasts, partly because they sought to avoid underestimating the number of passengers. Too low a forecast might have led to overcrowded trains, they said.