Metro officials were unable to explain yesterday why a Blue Line train derailed as it approached the National Airport station Wednesday night.

A switch that the train passed over was found to be slightly open after the train passed over it, but Metro officials said they could not tell whether the switch was improperly set before the derailment or if it was moved by the train as it went off the track. A Metro spokesman had initially said the derailment was caused by a switch malfunction.

No injuries to the 25 to 30 passengers were reported, according to Joe H. Sheard, director of rail services for Metro. Evacuation of the train took about 14 minutes from the time it went off the tracks, according to Sheard. Although the evacuation procedures will be examined as part of the accident investigation, Sheard said that he thought the passengers were taken off the train "in a very professional and very expedited manner."

The train derailed at 8:25 p.m. just past the top of the incline that brings it from below ground to the raised platform at the National Airport station. Because maintenance crews were performing "unscheduled routine maintenance" in the area at the time of the derailment, the train was being operated manually, according to Joseph P. Greenway, general superintendent of systems maintenance for Metro. Two other trains had passed over the same point in the previous half hour without incident.

According to Greenway, the train was going no more than five to seven miles an hour. Ultimately, only half of one car was derailed. Greenway said the train operator stopped the train as soon as he felt the train had derailed.

Sheard said it was "not possible" that the train could have fallen from the elevated tracks.

Greenway said that service to National Airport, which was disrupted when the power was shut off to evacuate the train and to get it back on the track, was restored at 10:50 p.m. The train was repositioned on the track by 1:20 a.m. and moved off under its own power. The affected car was removed from service to determine if had been damaged and to see if any defect in it may have caused the accident, Sheard said.

Metro officials investigating the derailment will not have answers to several questions until computer tapes that record the performance of the subway system's mechanical system are decoded and analyzed. Sheard said he has asked for a report by next week, but would allow more time to ensure a complete investigation.

The only other Metrorail derailment this year was the accident near the Smithsonian and Federal Triangle stations on Jan. 13 in which three passengers were killed.

The National Transportation Safety Board sent an investigator to the derailment site to see if safety issues similar to those involved in the Jan. 13 accident were involved in the derailment. Beyond that step, a spokesman said, the board has not undertaken its own investigation.