Because of a missing line, a story in Friday's Washington Post inaccurately stated the number of people Metro expects to ride the subway when subway hours are expanded Sept. 25 to include Saturdays and weeknights. The article said that Metro expects 60,000 riders on Saturdays and about 9,100 riders each weeknight between 8 p.m. and 11 p.m., the hours of added service.

The Metro board formally approved week-night and Saturday subway service beginning Sept. 25 when it adopted its 1979 budget yesterday.

The action means that on Monday, Sept. 25, Metro trains will operate from 6 a.m. to 11 p.m. on weekdays instead of stopping at 8 p.m. as they now do. Beginning Sept. 30, Metro will operate its trains on Saturdays for 14 consecutive hours, although the exact times have not been chosen. There will be no Sunday service. Trains will run every 10 minutes on both the week-night and Saturday schedules.

Some late night and Saturday bus service that parallels the subway lines will be cut back.

The added subway service will cost area governments about $2 million in subsidies to the Metro system, assuming the subway attracts as many paying customers at night and on Saturdays as the planners predict.

The planners expect about 60,000 people to ride the trains on Saturday and 9,100 people to ride between 8 p.m. and 11 p.m. each week-night.

Since the Metro extension to Silver Spring opened Feb. 6, the subways daily ridership has been climbing steadily. In the weeks before the Silver Spring extension opened, about 143,000 people a day had been riding the trains. Now, after more than two months of that service, the average daily subway ridership is 183,000. It has exceeded 193,000 on several occasions.

Although the board voted unanimously to approve the $191 million operating budget for the combined bus and rail system, the cost of late night and Saturday subway service if of concern to the local governments that support Metro.

Francis B. Francois, Metro board member from Prince George's County, sounded the warning yesterday. "We need to keep a close watch on the extended service," he said. "If the ridership isn't there, we want to (discontinue) it. The message is, if you want it, use it or lose it."

Metro expects fares and other revenues to pick up $93 million of the cost. About $98 million will have to be subsidized; $20 million of that subsidy will come from federal aid, the remainder from local budgets.