Updated

Hundreds of homes were evacuated Saturday after seven empty train cars and a tanker containing propylene derailed in a switchyard, exploding in a ball of fire and leaving a plume of smoke over the south end of the city.

One person was killed when a nearby home was destroyed, police spokesman Chris Rankin said. At least seven people went to hospital emergency rooms with complaints of respiratory problems, according to hospital spokeswomen. At least two homes and several vehicles were destroyed in the quarter-mile area surrounding the accident, Rankin said.

Seven hours after the 5 a.m. accident, the propylene tank continued to burn but the fire was under control and the smoke had thinned out, Rankin said. Still police were asking residents to continue to stay away from their homes.

"We're remaining cautious, but they can return home at their own risk," the officer said.

Initially, police thought the chemical involved was vinyl acetate (search), which releases poisonous fumes. Officers went door to door, urging thousands of people in a 2-by-5 mile area to move to the north side of town.

"The smoke was so thick it blocked out daylight. You couldn't see anything," Rankin said. "This is without a doubt the largest incident we've seen around here in at least a decade."

After arriving from Omaha, Neb., Union Pacific (search) spokesman Mark Davis said none of the railroad crews were injured.

Davis said a Union Pacific train coming from Chicago hit the back of another freight train in the rail yard, causing the eight cars to derail. He said his initial report that "a small LPG tank" was involved was erroneous. Union Pacific was investigating the cause of the accident.

The train was headed for Laredo, Texas, when it hit the back of the other Union Pacific train, which was coming from Pine Bluff and headed for Harlingen, Texas, according to Davis.

Rankin said the switching yard is behind the police station, about a quarter mile from the Texas border and also serves Kansas City Southern (search) and Amtrak (search). A park is nearby.

The evacuation area also includes one of two hospitals in the region. Wadley Regional Medical Center on the Texas side of the city treated six people who came in complaining about respiratory problems, spokeswoman Shelby Brown said.

And several of the hospital's 110 patients and some nurses complained of irritation to their noses and eyes so the hospital turned off its air conditioning to keep any fumes out of the building, Brown said. The hospital is about a mile from the accident site.

At Christus St. Michael's hospital on the Arkansas side of the city, only one person came to the emergency room hours after the fire with possible exposure to the fumes, hospital spokeswoman Clare Donohoe said.