If Metro has had one recurring problem in its 15 months of subway operations that fouls up schedules and irritates riders, it is the problem of a door that will not close. It happened again yesterday.

A six-car train on the new Blue Line from National Airport to RFK Stadium spent one hour traveling seven city blocks - the distance between the Eastern Market and Potomac Avenue stations - becauone of the doors on the sleek $300,000 Rohr Industries subway refused to behave.

Train 211 lurched through the tunnel a few yards at a time. Everytime the train would start to move, the door would signal that it had come open, and the brakes would be applied automatically. The continuous clacking of electrical relays filled the train with the odor of an electrical fire, although there was none.

The odor and a light blue haze of smoke were "very unpleasant," in the words of Harvey Weiss, a passenger.

The problem with Train 211 was one of three major technical problems in the metro system yesterday - the third day of regular operations on the 12-mile-long Blue Line.

A sump pump quit working overnight in the Federal Triangle station and water flooded the train control room and knocked out the signals on the track through that section until early yesterday afternoon. Metro ran the trains under manual controls with strict speed limits through that section until repairs were made and automatic trains operations could be resumed.

In the early afternoon, a section of track leading to National Airport lost power. Trains were run in on the other rail, but the alternate one-way operation cut into schedule time.

Other problems were not technical at all. Three stations opened late - most notably at Rosslyn where, according to Metro spokesman Cody Pfanstiehl, the station attendant simply did not report for work by the scheduled 6 a.m. opening. A supervisor got the station open at 6:45, to the disgust of about 30 to 40 waiting people, accordint to Pfanstiehl.

Problems with the locks on the gates kept on-time attendants from opening the Eastern Market and Foggy Bottom-GWU stations until 6:15 and 6:30, respectively, Pfanstiehl said.

The good news was that the trains ran more closely on schedule through most of the day - and through the evening rush hour - than in either of the preceding two days. Although Metro did not have a passenger count immediately available, it was clear from observations taken throughout the day that there were fewer joyriders than there had been on Friday or on the July 4 holiday.

On July 4, Metro collected $51,506 in 50-cent fares, which would translate to 103,012 people. There were many more trips than that, however, because literally thousands of people rode from one end of the line to the other, then just walked across the platforms to take the return train without paying another fare. The crush of sightseers at National Airport was so large that, at one point, Metro just let them through the gates free to clear a people jam.

Yesterday, with the electronic fare-collecting equipment back in operation, people seemed to be getting the hang of using Farecard - the electronic ticket they must purchase. Employees of Cubic Western, the company that manufactures the equipment, had been working on Farecard vending machines and the entrance-exit gates throughout the weekend to improve on the opening-day performance on Friday, when Farecard was a bust.

There were some problems, but Metro employees had had a day of experience to deal with them. At Rhode Island Avenue on the old Red Line, for example, there were no long lines as Metro officials sold pre-encoded $1 cards to disperse the commuters tumbling off the buses and out of their cans there. There had been near-chaos there on Friday.

Because of the reduced crowds - and more six-car trains on the Blue Line (four cars had been the norm - there were fewer standing-room-only trains and thus fewer door problems.

The two are related, whether they should be or not. Metro's cars were designed to be as light as possible and still meet the needs of mass transit. Weight translates into money: the heavier the car, the more electricity it takes to propel it.

"The traditional concept was to build a platform, put a bird-cage of steel ribs on the platform, then rivet the sides to the birdcage," said Larry Peeples, spokesman for the Rohr Industries, builder of the metro cars.

"The technique used on Metro is called semimonoque - which means simply that the walls and sides of the car bear a large portion of the stress themselves."

What has happened, according to David Gaul, Metro's equipment design chief, is that standing loads on the trains have caused the body of the car to flex slightly - thousandths of the inch. Such flexing is predictable and expected, but it is transmitted to the tracks in which the doors travel and makes them fractionally smaller.

Thus the doors bind on closing and do not provide a solid electrical signal that they are closed. Automatic fail-safe devices prevent the train from moving while the door is open.

This problem first cropped up on the first day of Metro's operations on the Red Line, when thousands of area residents stood in long lines on a Saturday for a free ride. The trains would not move.

"We made major changes in the frame on which the doors rest after that experience," said Gaul. "We though we had the problem licked."

The design specifications, he said, call for an individual car to carry an "absolute maximum load of 240 passengers," which is more than anyone has seen on any of the train cars so far, including the full ones.

Does that mean there is a mistake in design - a design problem?

"I don't think it is s design problem," Gaul said, "but I can't sit here and swear that it isn't . . . I think adjustments can be made to carry these louds."

Metro has taken delivery on about 150 cars out of an order from Rohr of 300 for a total fixed-price contract of $93.4 million. Rohr has estimated that its losses on the contract total $45.8 million, and it has filed a $48.3 million claim for additional compensation against Metro.