The train continued burning Saturday morning and officials said they would wait for the "very dangerous" inferno to burn out by itself.
Rockford Fire Chief Derek Bergsten said 74 of the train's 114 cars were filled with ethanol, or ethyl alcohol.
Officials evacuated the area on the edge of Rockford, about 80 miles northwest of Chicago, Friday night amid concerns about air pollution and the chance that more of the train's cars might catch fire.
Winnebago County Coroner Sue Fiduccia said early Saturday the death was that of a female who was in a car waiting for the train to pass a crossing near the derailment site.
Bergsten said three other people ran from the car when it was bombarded with flying railroad ties and they were severely burned by flaming ethanol. 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.
Two crewmen on the eastbound Canadian National train escaped injury, said company spokesman Patrick Waldron. The engine crew was able to pull 64 cars away from the scene.
The cause of the derailment was still under investigation Saturday but witnesses told the Rockford Register-Star that cars on the Chicago-bound train began hydroplaning in standing water as it approached the crossing. Some of them left the tracks moments before two of them exploded.
Parts of northern Illinois may have gotten up to 4 inches of rain Friday, said meteorologist Gino Izzi of the National Weather Service. Chicago's O'Hare International Airport, 40 to 50 miles east of Rockford, measured 3.6 inches, a record for the date, he said.
Kirk Wilson, a fire chief in nearby Rockton, said he expected the ethanol to continue burning until later Saturday.
"We're letting the product burn itself out," he said. "We can't get too close to it. We're observing everything through binoculars from about 200 or 300 feet away."
