An engineer ignoring a "slow order" apparently caused a train to derail, starting a fire in a phosphorous tank car that sent a toxic cloud over eight counties and forced 15,000 people to flee their homes.

Authorities were forced to allow the car carrying poisonous phosphorous to burn itself out after the derailment about 7:30 p.m. Wednesday because of the danger of an explosion, said Don Armstrong, a spokesman for the state Disaster and Emergency Services. The fire ended about 6 a.m. Thursday."The cloud has dissipated," Armstrong said. "As of a 9 a.m., 4,000 to 5,000 people remained out of their homes at various areas."

He said about three dozen people suffered minor eye irritations, breathing difficulties, and other ailments related to the cloud "but nothing serious."

Armstrong said as many as 15,000 people were evacuated overnight from an eight-county area - Christian, Hopkins, Ohio, Muhlenberg, McLean, Butler, Grayson and Breckinridge counties.

Armstrong also said a ban on low-level flights up to 5,000 feet, 3 miles out from the point of the derailment, was reinstated at 9 a.m. as a precaution.

Authorities had closed 50-mile stretches of both the Western Kentucky Parkway and the Green River Parkway and a part of the Pennyrile Parkway in Christian County.

"All highways closed in the path of the cloud were reopened at 4 a.m.," said Kentucky State Police Lt. Don Weedman.

Thirty-six cars of the 80-car CSX train traveling from Nashville, Tenn., to Evansville, Ind., derailed 3 1/2 miles north of Crofton, a CSX spokesman said. It ignited one tank car - the only one carrying phosphorus, a chemical used in military explosives, Armstrong said.

"It appears the engineer was going 35 miles per hour under a 10 mile per hour slow order," said CSX spokesman Lloyd Lewis. "A previous train had thought there might be something wrong and they were checking it out when the cars derailed."

Before dawn, Armstrong said the plume was wafting 3,000 feet high and stretched for 50 miles.

"Phosphorus fumes are extremely toxic. It's very, very dangerous. It's toxic to inhale and it burns the skin," he said.