Motor failures and a shortage of spare parts have been grounding between one-third and one-fourth of Metros subway cars on recent days, thus putting a severe strain on the system at a time when foul weather is attracting record numbers of riders to Metro.

Veteran riders find themselves waiting longer for trains than usual, then find that the trains are jammed full of people when they arrive. On several occasions in the past few days trains have left passengers on the platform because three was no more room inside.

Trains that are usually eight-cars long -- the maximum for Metro's stations -- have been reduced to six cars. Intervals between trains have been increased from five minutes to six or even eight minutes.

Yesterday, for example, a Metro regular who transfers from the bus at the Pentagon and takes the subway to Farragut West on the Blue Line found that a normal 20-minute transfer and ride stretched to 50 minutes.

"We're short, there's no question about it," said Nicholas A. Roll, Metro's assistant general manager for transit services. "If we had the normal patronage, it would have been no problem, but that's not an excuse, just a statement."

Metro has 288 cars that are theoretically available for service and would like to have 250 cars for rush hours, including 14 spares.That means that 87 percent of the fleet should be available at rush hour.

Last Wednesday only 200 cars ran. There were no spares and many trains ran with fewer cars than normal. On other recent days, Metro has been running with 216 to 226 cars, according to sources.

Meanwhile, a combination of tractor blockages and lousy weather has encouraged thousands of people to try Metro instead of their cars to get to work. Last week, Metro averaged 259,000 riders per weekday on the subway, including the record first day of the farmers' protest, when 270,000 people tried the subway. Before the farmers and the snow, Metro was carrying a weekday average of about 225,000 riders.

In Metro's repair shops, there is a "procurement problem," to use Metro general manager Theodore C. Lutz' term. Metro has sent out a rush order for brushes, those vital components that make electrical motors work.

Privately, Metro officials call the supple line "archaic." Lutz said "I think we need to improve our sophistication in the whole purchasing area." In any event, the brushes were not ordered in time to ensure that they would be there when needed.

Metro has 1,200 motors for its subway cars, one for each axle. Sixty motors are out of service right now, tying down at least 20 cars. A variety of other problems, from routine maintenance schedules to inoperative heaters, have grounded the rest of the missing cars.

Metro and Westinghouse, which manufactures the motors, have been seeking solutions to a motor failure rate of 4 to 5 percent that has begun to crop up as each subway car reaches 60,000 to 70,000 miles. Before that, the rate was less than 1 percent. A new brush replacement schedule is part of the solution, officials for both Metro and Westinghouse said.

Bearings are also failing prematurely on the motors and a study is under way to determine if different maintenance can improve that."As far as we can tell, almost all the failures are due to normal wear and tear," Westinghouse's Joe Iannotta said.

Metro's Fady P. Bassily said "We still don't have enough information to really analyze" the failures. Metro's car-availability percentage (about 75 percent) is lower than that of Chicago (89 percent) and Philadelphia's Lindenwold Line (94 percent), but about the same as that in New York City.