Metro officials yesterday blamed human error -- an improperly operated switch -- for the derailment of a train near National Airport the evening of Sept. 1.

The five-member committee that investigated the derailment said in a tersely worded report to Joe H. Sheard, Metro's director of rail service, that a switch near the National Airport station was "not fully mechanically locked and properly blocked, allowing switch points to open" before the train began passing.

At the time the derailment occurred, a three-man crew was performing routine maintenance at the site, operating the switch in question manually. The investigation established that the switch itself was operating properly after the derailment. The investigating committee blamed the crew for improper operation of the switch.

The investigation also determined that the derailment had caused $3,300 in damage, including $1,700 to a rail car, $1,400 to equipment and $200 to the track.

Although there were no reported injuries, it was the second time this year that switching problems and human error caused the derailment of a Metro train. An improperly blocked switch precipitated a chain of events that led to the derailment of a train on Jan. 13 in which three persons were killed and 25 were injured.

Sheard said in a cover memo accompanying the report that he was referring it to Joseph P. Greenway, Metro's general superintendent of systems maintenance to determine "what appropriate follow-up action is required."

Greenway said there "could well be" disciplinary action taken against the maintenance crew, but he would not speculate on that question until he received recommendations from a subordinate about what corrective steps he should take. Greenway said he was "sure" the corrective measures will involve retraining of maintenance crews to prevent a recurrence.

Some 25 to 30 passengers were aboard the Blue Line train when the derailment occurred. Metro officials reported that the passengers were off the train within 14 minutes of the derailment and that Metro's procedures had been followed in the evacuation. The investigating committee made no comment about the evacuation.