Until 4:30 p.m. last Wednesday, Metro officials boasted that they ran the safest subway system in the country.

Without a single passenger fatality in the system's nearly six-year history, officials tended to downplay the significance of occasional rail mishaps. They also paid little heed to the warning of critics who said Metro was headed for a disaster unless it improved its safety features and stepped up contingency planning for emergency evacuation of passengers.

"They always had an 'it-will-never-happen-to-us' attitude," said William Gossard, a staff member of the National Transportation Safety Board (NTSB) who has specialized in subway accidents. "They have been somewhat arrogant."

But then a six-car Orange Line train derailed in a freakish accident near the Smithsonian station on Wednesday, leaving three passengers dead and 25 others injured, and badly shaken Metro officials acknowledged they would have to reconsider some of their policies.

"I think we're going to learn quite a bit from this experience," Metro General Manager Richard S. Page said in an interview last week.

Two investigations independent of Metro's own were begun to untangle events leading up to the fatal derailment and to assess the response by Metro employes and D.C. fire rescue units, who took more than 2 1/2 hours to evacuate the estimated 1,200 to 1,500 homebound passengers on the ill-fated train. The investigations are being conducted by the NTSB, a long-time critic of Metro safety standards, and the American Public Transit Association.

Rescuers were confronted with a nightmarish situation. The streets and subway were jammed with government workers who had been let off early that afternoon because of a snowstorm. The train crash occurred only a half hour after an Air Florida jetliner crashed into the nearby Potomac River, killing a total of 78 people and drawing the lion's share of area rescue personnel and equipment.

The investigations are centering on possible human errors that may have triggered the disaster. But NTSB investigators also want to know whether the response of Metro employes and D.C. fire units could have been improved through better planning, whether passengers were provided information over the train's intercommunications system soon enough to avert panic, and whether one of the three victims, Mariano Cortez, might have lived if rescuers had gotten to him sooner.

Cortez, 41, of Riverdale, Mildred M. Morgan, 71, of Hillcrest Heights, and Mary L. O'Meara, 25, of New Carrollton, apparently were killed when the left side of the car they were riding in crashed into an abutment as the train was being backed up just north of the Smithsonian station at 12th Street and Independence Avenue.

Cortez died of a compressed chest, according to deputy D.C. Medical Examiner Stuart Dawson, although Dawson couldn't say whether Cortez died in the subway or on the way to the Washington Hospital Center. Initial reports said Cortez died shortly after the accident.

An eyewitness told NTSB investigators that Cortez "seemed alive" 40 minutes after the crash, while still trapped in the mangled car. Page, the Metro official, said he was told by fire officials that Cortez had died in an ambulance. Yet Cortez was not pronounced dead until 8 p.m., four hours after the accident, according to his death certificate.

"That's one of the things we're looking at," said Patricia Goldman, an NTSB board member who is supervising the investigation.

Goldman also is concerned that Metro may have been slow in alerting the fire department to the accident. "There is some indication that the first notification to the fire department was from a fire box, not from Metro," she said.

Moreover, NTSB officials noted that the death toll might have been substantially higher had a fire broken out during the time-consuming rescue and evacuation operation.

For years, fire officials have complained that Metro employes have been lax or careless in responding to fires. A classic case of this occurred on April 16, 1980, when Metro officials delayed notifiying the Arlington Fire Department of a small fire that filled a 1.2-mile tunnel under the Potomac with smoke. When fire crews finally arrived, they were told that a Metro supervisor had ridden several trains filled with passengers through the tunnel to check on reports of smoke.

Fire officials also have repeatedly warned that, if ignited, the polyvinylchloride plastic used in the interior walls of Metro's cars is highly toxic, and that the attractive orange-cushioned seats can easily catch fire.

Page said Metro officials meet regularly with area fire chiefs to discuss ways to improve fire-prevention and evacuation techniques. Even before the accident, under pressure from fire officials, Metro began to install a new two-way communications system to coordinate future subway rescue operations. Also, Metro and fire departments stage full-scale evacuation drills whenever a new Metro station is opened. Two such dress rehearsals were held at the new Van Ness Center station last month.

But Goldman, whose agency conducted extensive hearings into Metro's safety standards in July 1980, insisted last week that D.C., Virginia and Maryland fire departments weren't getting enough "hands-on" experience in laying hoses and trying to free "victims" of subway disasters.

Moreover, NTSB members for years have disagreed with a Metro policy preventing passengers from abandoning trains in the tunnel in the event of a derailment or fire. Currently, there is no way for a passenger to stop a train in an emergency or force open a car door and flee through the tunnel. Metro officials have insisted that it is safer for passengers to remain in a disabled train until the train can be moved to the nearest station or until crews cut off power on the track, which carries enough voltage to kill someone, and release passengers from the cars.

But given the seemingly interminable delay before passengers were freed from the derailed train last Wednesday, Page said the Metro board would have to rethink its policy.

"It's a close question and reasonable people will differ," he said. "So far it's been our opinion that the safest position is not to let a citizen shut down the railroad . . . get out of their cars and walk down the track, where it's more dangerous. Obviously, this accident Wednesday night is going to cause us to review that policy."

Even the structure of the subway cars may undergo renewed scrutiny, although Metro has already bought 300 of the light-aluminum units from Rohr Industries Inc., an air frame manufacturer.

The cars are more fragile than the ribbed, stainless steel cars used in other subway systems, including the new one under construction in Baltimore. The side of the Metro train that struck the concrete divider last Wednesday caved in and crushed the three victims. The train was traveling 2 to 3 miles an hour at the time.

In discussing last week's crash, Metro officials stressed that the system's overall performance record has been excellent. Repeatedly they cited a 1979 U.S. Department of Transportation study indicating that the number of accidents involving Metro trains with passengers aboard was one-sixth the industry-wide average.

Metro officials said the handful of serious injuries and fatalities that occurred here before last Wednesday's incident primarily involved people tripping on escalators, platforms or in subway cars or suicides who jumped in front of oncoming trains.

During the past six years, derailments, rear-end collisions and fires have been the major cause of serious subway accidents around the country investigated by the NTSB, an independent federal investigative body.