Metro officials announced plans yesterday to increase the number of cars on some rush-hour subway trains on the recently expanded Red Line in an attempt to reduce crowding.

Starting Monday, all Red Line trains will consist of six cars, officials said. Because of car shortages, the transit authority recently has operated a mixture of four-car and six-car trains on the line.

The four-car trains appear to have been the main cause of rush-hour crowding, officials said. Passengers waiting near the ends of a station's platform must walk farther to get aboard the four-car trains. So these trains frequently are delayed and eventually may become crowded.

The use of four-car trains "is killing us. It's overcrowded and it's dragging the line," said Fady P. Bassily, Metro's assistant general manager for rail service. Bassily said the shift to six cars would ease crowding and also help keep Red Line trains on schedule.

When the Red Line was extended to Montgomery County's Shady Grove terminus in December, Metro officials had warned that commuters might face some crowding because of rail car shortages. An expansion in bus service this week to the new Red Line stations also has increased subway ridership, officials said.

Crowding on the Red Line is expected to be reduced further next month under Metro's previously announced plan to increase rail service. Starting March 18, trains are to run every three minutes instead of every four minutes on most of the Red Line during rush hours.

Overcrowding recently has prompted complaints from some Red Line passengers. Bassily said the authority's surveys showed that four-car trains frequently were jammed with as many as 200 riders in each car, a crowd he described as "quite uncomfortable."

Sometimes, he added, the four-car trains have been so packed that they "are actually leaving people on the platform" because no more riders can squeeze aboard.

Transit officials repeatedly have cited delays in rail service as a key factor in crowding throughout the Metro system. On the Red Line, Bassily said, the four-car trains tend to be delayed because many passengers, accustomed to boarding six-car trains, wait too far from the middle of a platform.

Metro officials said they would provide 25 six-car trains on the Red Line next week, instead of the current mixture of eight four-car and 17 six-car trains, by putting some newly delivered cars into service and temporarily reducing spare cars in the rail system's reserve fleet.

When Red Line service is increased next month, Metro plans to return to four-car trains. The increased service is expected to offset the shorter trains, officials said.