Some rush-hour subway trains have become severely crowded because the Metro system has failed to keep its trains on schedule, according to a report released yesterday by the transit authority.

Twenty-one excessively crowded trains were cited by Metro officials in a survey taken at several heavily used rail stations on two days last month. Seven of these trains were so packed that no more passengers could get aboard without shoving.

"A program must be developed to improve schedule adherence throughout the system," the report concluded. If trains cannot stay on schedule, the report added, additional subway cars "must be provided" to reduce overcrowding.

Theodore G. Weigle Jr., Metro's assistant general manager for transit operations, said several steps already have been taken, including stationing supervisors at key posts to help keep trains on time. Cars on some Orange Line trains also were added recently to ease crowding.

When a delay occurs at rush hour, a crowd may gather at a station to wait for a train. When a train eventually arrives, it may be delayed further because the crowd requires more time to squeeze aboard, officials said. These delays may exacerbate crowding at stations farther along the route.

Weigle said two other factors contributed to the crowding and delays cited in the report. Some trains get off schedule when they turn around at a terminus to begin another trip because of "poor line discipline," he said.

In addition, Weigle said, the rail system does not have automated equipment to ensure that trains are on time. It is difficult, he said, for control room personnel to monitor trains closely enough to avoid delays. Officials now are considering new control room equipment that may eventually remedy this problem, Weigle said.

One instance of overcrowding cited in the report occurred at the Foggy Bottom-GWU station on the Orange Line at about 5:15 p.m. Sept. 12.

Three trains in a row were jammed with an average of more than 190 passengers in each car. One train averaged 200 passengers; the next had more than 205; and the third, 195. The crowding was attributed to "a delay in service at a critical time."

A subway car with 190 passengers is "fully loaded," according to the report. "This is the general point beyond which passengers will not attempt to board a car."

Metro officials have sought to maintain an average of 120 to 160 passengers per car during rush hours, with any larger number considered in excess of the "optimum" range. Most Metro cars have 80 seats, although Metro's new Italian-made cars have 68. A total of 229 passengers is considered a "crush load," a maximum reached only after a football game, July 4 or similar events.

The 21 instances in which average car loads exceeded 160 passengers in the Sept. 12-13 survey included a Red Line train at Judiciary Square (185 riders), two Red Line trains at Dupont Circle (over 160 and nearly 180), two Yellow Line trains at the Pentagon (180 and over 190), an Orange Line train at Court House (170), an Orange Line train at Federal Center SW (175), three Orange Line trains at L'Enfant Plaza (nearly 180 to almost 200), five Blue Line trains at the Pentagon (nearly 180 to 210), two Blue Line trains at Foggy Bottom-GWU (nearly 210 and 170) and one Blue Line train at Federal Center SW (slightly over 160).

The report stemmed from recent recommendations by some county and city officials to reduce rail service to cut costs. Metro officials said current service levels are adequate for the most part and did not suggest cutbacks.