Residents of this suburb of Dayton began reoccupying their city today, two days after a railroad tank car loaded with white phosphorus derailed near downtown and caught fire, sending up clouds of noxious gases that drove thousands from their homes.

Although the wrecked car continued to burn, generating new clouds of white, irritating fumes, state and local officials said the danger that led to back-to-back evacuations Tuesday and Wednesday nights had passed. Officials said it was the largest evacuation resulting from a U.S. train wreck: more than 17,000 were evacuated Tuesday and as many as 40,000 last night.

Authorities said that all but a few hundred families in Miamisburg (population 18,000) could return today, but residents of areas near the wreck were told to stay away until the fire is out for good.

Parts of the city were declared an "irritant zone." Residents -- especially children, the elderly and those with respiratory problems -- were advised to exercise "extreme caution" in deciding whether to return.

Hospitals reported treating more than 400 people complaining of shortness of breath, irritated eyes or sore throats. At least 11 people remained hospitalized, and one death -- that of a 34-year-old evacuee whose camper caught fire -- was reported.

The tanker was partially opened late tonight to force the white phosphorus, a toxic chemical that ignites spontaneously at 85 degrees, to burn more quickly. A fan was installed to feed additional air to the fire, which emitted a larger plume of smoke. Dale Hawk, an official with the company that operates the railroad, estimated that the fire would continue at least until Friday morning.

The tank car carried 12,000 gallons of phosphorus in a sealed container, and officials were uncertain today how much remained. Estimates ranged from 100 gallons to 1,000.

The derailment occurred Tuesday evening as a 44-car freight train was southbound across a trestle bridge over Bear Creek, in the valley of the Great Miami River, which courses south toward the Ohio River along Miamisburg's western edge. The trestle carries the main line of the CSX Railroad from Dayton to Cincinnati. Fifteen cars of the Southland Flyer derailed, and the tanker of phosphorus, the only one in the train, burst into flames.

A west wind blew the fumes from the billowing fire over this city, described by councilman Paul Gutshall as "a very middle-class community," and within minutes an evacuation of four communities was under way.

The cause of the derailment is under investigation.

A CSX spokesman said the track was inspected a few days before the accident. Other company officials have said there is no indication that the four-member crew was at fault, or that the train was speeding.

A $200 million class-action lawsuit was filed in U.S. District Court in Dayton today on behalf of four residents who charged the railroad with negligence and operating at excessive speed. CSX officials were not available for comment tonight.

As the initial evacuations took place, firefighters poured up to 5,000 gallons of water a minute on the burning phosphorus car. By Wednesday it appeared they had put the fire out, and the 17,500 evacuees began returning. But about 6 p.m. the phosphorus exploded, releasing a new cloud of poisonous fumes and prompting a second evacuation, this one affecting 20,000 to 40,000 people.

At noon today, Miamisburg resembled the set of a science-fiction movie. Empty houses lined empty streets. No children played in the yards. No bicycle riders sped on the sidewalks. No oldsters strolled in the sun. No vehicles moved anywhere, save police cars at blockades and the ever-present, ever-curious news media.

Out on Interstate 75, the exit to Miamisburg was marked "Closed."

Norma Wolff, a city finance department cashier, said she and her husband and two children spent Tuesday night in a local shelter but headed for Cincinnati, 50 miles south, after Wednesday's flare-up.

"It's something we hope never to see again," she said.

For Miamisburg residents, this week's episode was overshadowed by the memory of a derailment several years ago on tracks that run through the heart of town. That time the rail cars plunged off the tracks and demolished several houses, killing four people.

"It made toothpicks out of the houses," Gutshall recalled