A Metro supervisor who was in charge of the subway train wrecked in last week's fatal derailment has told federal investigators that he failed to inspect the train's front wheels to make certain it could be backed up safely, a key U.S. safety official said yesterday.

The accident, in which three passengers were killed, occurred as a crowded Orange Line train was being operated in reverse over a rail crossover switch near the Smithsonian station. Federal investigators now say the supervisor's failure to examine the front wheels appears to be a key factor in the derailment, the first fatal wreck since the subway system opened nearly six years ago.

"Really, what matters is he didn't do it. He didn't determine the exact location of the wheels of the car before he went to the rear to back up the train ," said John Rehor, a railroad specialist in charge of the National Transportation Safety Board's investigation of the accident. "The supervisor from his vantage point didn't recognize that there would be a problem."

An experienced railroad operator would have readily recognized that a potential for derailment existed because of the position of the train's front wheels when the subway stopped in the crossover, Rehor said in an interview. The protruding rim of the right front wheel had become improperly situated on top of a rail, he said.

But he questioned whether Metro employes are sufficiently trained to recognize problems such as the one that existed immediately before the derailment, in which one rail car was crushed against a concrete divider.

"Metro operators are not about to get that type of training," Rehor said, voicing concerns similar to those of other safety officials. Patricia Goldman, the federal safety board member overseeing Rehor's investigation, said at a news conference earlier in the day that the training given to Metro subway workers is among the overall issues now under study.

Rehor's statements yesterday were the first detailed account of the actions of the supervisor who was at the train's controls when it derailed at 4:29 p.m. last Wednesday. The supervisor has not yet been identified publicly. "He didn't inspect any of the wheels. He didn't get any closer to the front of that train than 150 feet," Rehor said.

The six-car train bound for New Carrollton derailed in a tunnel shortly before reaching the Smithsonian station, at 12th Street and Independence Avenue SW. Officials say the train initially was halted after its front car entered a crossover leading to an incorrect track. The crossover switch had been adjusted manually--by the same supervisor who reportedly failed to check the front wheels--and investigators are still seeking to determine why the switch was improperly aligned.

The supervisor then went to the rear car of the train and took over the controls to try to get the subway back to the correct track. The derailment occurred almost immediately, with the train traveling at about 2 to 3 mph.

Federal officials say the front wheels followed a complex zigzag, first being drawn back into the crossover and then returning to the incorrect track. The rail car, dragged diagonally through the tunnel, was smashed against a concrete divider between the two tracks. In addition to those killed, at least 25 passengers were injured.

At her news conference, safety board member Goldman provided numerous other details. "No braking action was initiated from either end of the train," she said, apparently indicating that the crash itself stopped the subway. But she added that investigators have not determined whether the train could have been stopped sooner if the supervisor or the operator, who also has not been publicly identified, had tried to use an emergency brake.

She also said a plainclothes Metro police officer aboard the wrecked rail car radioed for emergency help within five or six minutes of the derailment. Safety officials say the presence of the Metro police officer was extremely lucky, especially because the rail car's intercom system broke down in the accident. Although yesterday's account indicated that rescue efforts may have been quicker than some reports previously suggested, Goldman noted, "It's much too early to make a judgment as to the adequacy of the response time of rescue workers ."

In other developments, D.C. Mayor Marion Barry and Police Chief Maurice T. Turner Jr. announced formation of a special task force to study the response of rescue agencies to the subway derailment and the Air Florida plane crash, which occurred the same afternoon. First Lady Nancy Reagan visited one of the passengers injured in the subway derailment during a visit to Washington Hospital Center.