A Metro train derailment at National Airport station that disrupted rush-hour service last Wednesday morning was caused by a train operator who ran through a red signal, the Metro board was told yesterday.

The operator was given a five-day suspension without pay and was returned to duty yesterday, Ralph L. Wood, director of the transit system's rail operations, reported. Wood declined to identify the operator.

The accident occurred at 6:55 a.m. June 20 when the eight-car train moved away from the National Airport platform and struck a safety device called a derailer, knocking the front wheels of the first car off the track.

None of the 100 passengers aboard was injured, but full service at the station was disrupted for four hours.

The train was being operated manually, as is normal at National Airport station, Wood said, rather than by the computerized automatic control that normally is used when trains are on the line.

If the train had been under automatic control, it could not have moved past the signal. Power to the motors would have been cut off and the brakes triggered. Manual control is used at National Airport because it is a jerry-rigged temporary terminal on a line that ultinately will extend through and beyond Alexandria, and because slow speed is required on crossover tracks because of worn rails, Wood said.

Wood said the operator should have waited for Metro's control center to flash a green signal. When the operator moved the train through the red signal, the derailer did precisely what it was supposed to do - knock the train off the rails, preventing a potential collision if there had been an obstruction of another train on the track.

Wood said it was the third instance since Metro began operations in 1976 that an operator was suspended for running a red signal.

At yesterday's meeting, the board was told that 232 cars were dispatched for service yesterday morning, compared with 200 one week previously.

To maintain full scheduled service on both lines, 236 cars are dispatched each weekday and eight others kept in reserve. Last week, more than 40 cars were sidetracted because of excessive wear of parts called "caliper links" in the brake assemblies of the cars.

Repairs to the cars awaited shipments of spare parts, which are covered by warranty, from the manufacturer. A total of 16 caliper links, each valued at about $100, are installed on each car.

Also at yesterday's meeting, the Metro board placed a formal order for 94 more subway cars at a total cost of $75.3 million - an average of $801,000 for each car, including a stock of spare parts.

The cars will be manufactured by Breda Construzioni Ferroviarie of Pistoia, Italy, and will be assembled at a plant near Washington. Delivery is scheduled to start in 23 months. The Italian firm submitted the lowest of three bids in May.

Metro's existing fleet of 300 was manufactured at a cost of about $300,000 per car in Georgia by Rohr Industries, which has gone out of the transit car business.

The new cars will look the same as the present Metro vehicles on the outside, but will have 12 fewer seats to accommodate more standing passengers, and will be arranged differently inside the doors to reduce crowding.

Metro board members also told staff officials yesterday to take steps to assure that passengers on stalled trains, as well as those waiting on platforms, are given an explanation for delays.

Jerry A. Moore Jr., the Metro board chairman and a D.C. city councilman, complained that he was on a train one day last week that made a stop-and-go trip that took 27 minutes from downtown to National Airport. He said he almost missed his plane.

Passengers were evacuated from the train at Crystal City, without explanation, Moore said, and transferred to another.

Wood, the operations director, said later that a check of the train log for the day cited by Moore indicated no unusual operating problems.