An early-morning Metro train derailed yesterday as it was leaving the National Airport station, disrupting service for about four hours but causing no injuries.

The accident occurred at 6:55 a.m., Metro spokesman Cody Pfanstiehl said, when a safety device called a derailer knocked the first car of an eight-car train off the tracks.

The train was moving so slowly, Pfanstiehl said, that none of the 100 passengers aboard was even badly shaken. They were able to leave the train, he said, through cars that remained beside the platform, and were taken by bus to the next station on the Blue Line at Pentagon City.

Limited service from the airport station resumed about 7:45 a.m., Pfanstiehl said. Full mid-day service was operating by late morning, he said, but the derailed car was not removed until about 1 p.m.

Pfanstiehl said Metro officials have not determined why the derailer was activated. He said the device - a steel arm that moves across the track - is supposed to function only when there is danger of a head-on collision, and that no such emergency existed yesterday.

While service was curtailed at National Airport, Pfanstiehl said, most trains turned around at the Pentagon City station, causing serious problems for some commuters but providing seats for many on the line who normally are forced to stand.

The line runs from National Airport through downtown Washington to New Carrollton in Prince George's County. CAPTION: Picture 1, Metro officials inspect tracks near National Airport yesterday morning after derailment that blocked rush-hour service to the Blue Line terminal. Photos by James A. Parcell - The Washington Post; Picture 2, Workmen move a metal beam to the derailment scene to aid in repair of blocked Metro line; Picture 3, Derailed car came to rest across curve in track.