The number of subway riders getting on and off Metro trains at Montgomery County's new Red Line stations has climbed to more than 18,000 a day, according to a report released yesterday by the transit authority.

Officials said ridership at the four stations, which opened Dec. 15, appears likely to reach Metro's preliminary target of 20,000 trips a day within a few weeks. "I think we'll hit it and perhaps exceed it," said Harold W. Barley, a senior rail analyst. "So we're in good shape."

The number of passengers getting on at the White Flint, Twinbrook, Rockville and Shady Grove stations rose from an average of 7,860 on Dec. 18 and 19 to 9,100 on Jan. 3. On most days, the number of passengers getting off at the new stations is about the same as the number getting on, so total ridership was estimated at about 18,200.

Officials said ridership appeared to have been held down because of the Christmas and New Year's holidays. "Substantial increases are expected over the next several weeks as people return from holiday vacations and as schools reopen," the report said.

A further influx of riders is expected after bus routes are revamped later this month. On Jan. 27, the Metrobus system and Montgomery County's Ride-On bus system are scheduled to increase service to Red Line stations.

Metro officials previously predicted that ridership at the new Red Line stations would climb from 20,000 trips a day in late January to 27,000 after bus routes are revised.

If ridership exceeds Metro's forecasts, officials said, passengers may face some crowding on rush-hour trains next month because of Metro's continuing shortages of rail cars. "It's going to put some pressure on the amount of seats available," said Deputy General Manager Theodore G. Weigle Jr.

Many parking spaces are still available at the new stations, officials said. A Jan. 3 count found that 3,165, or 56 percent, of the 5,620 spaces in Metro lots at the four stations were filled.

Edward A. Daniel, transit services chief for Montgomery County, said the Rockville lot had been full in recent days, but that spaces were available at the larger White Flint, Twinbrook and Shady Grove lots.

The report said that during midday and evening hours and on weekends, "a considerable amount of excess capacity" exists because of a recent increase in nonrush-hour service.

"Although the level of [nonrush-hour] service is certainly more attractive from the passenger's standpoint," the report warned, Metro officials "will be looking carefully at whether this [increased service] is warranted."