Three other people from the same car escaped with severe burns. Hundreds of people were evacuated from homes near the explosion.
Eighteen tank cars, all filled with ethanol, or ethyl alcohol, derailed Friday on the edge of Rockford, about 80 miles northwest of Chicago.
The wreckage burned through the night, but the fire was dying down Saturday afternoon as federal investigators began collecting evidence.
National Transportation Safety Board spokesman Robert Sumwalt said investigators wouldn't speculate about what may have caused the derailment. A thorough investigation could take a year, he said.
Investigators will look into the train crew's performance, the train's mechanical components, signals, the integrity of the cars containing ethanol and the emergency response, Sumwalt said.
Rain possible cause
Reports that the derailment was caused by a washout of the tracks following heavy rain were "not a certainty and this remains under investigation," said Canadian National Railway Company spokesman Patrick Waldron.
The woman who was killed -- Zoila Tellez, 41, of Rockford -- had escaped from the stopped automobile, but she managed to get only 20 feet away before she fell and died, said Winnebago County Coroner Sue Fiduccia. Tellez' body was found face down on the ground.
Three people with the woman also ran from the car when it was bombarded with flying railroad ties and they were severely burned by flaming ethanol, said Rockford fire chief Derek Bergsten. They were taken to OSF Saint Anthony Medical Center in serious to critical condition, and one was transferred to Stroger Hospital in Chicago, he said.
Officials evacuated about 600 homes in the area on the edge of Rockford, about 80 miles northwest of Chicago, Friday night amid concerns about air pollution.
Some people were allowed to return to their homes Saturday, but authorities were not letting anyone within a quarter-mile of the derailment site, said Cherry Valley Fire Protection District battalion chief Allen Geeser.
Two crewmen on the eastbound Canadian National train escaped injury, Waldron said. The engine crew was able to pull 64 cars away from the fire.
At least 26 fire departments had sent crews to the scene.
Witnesses told the Rockford Register-Star that cars on the Chicago-bound train began hydroplaning in standing water as it approached the crossing.
Parts of northern Illinois may have gotten as much as 4 inches of rain Friday, said meteorologist Gino Izzi of the National Weather Service. Chicago's O'Hare International Airport measured 3.6 inches, a record for the date, he said.