CSX, the transport company operating the train, said two of the at least 16 cars affected were carrying molten sulfur.
Specialized equipment was being deployed to monitor the air for sulfur dioxide, a toxic gas that is released when molten sulfur burns, the company said.
It was not immediately clear what caused the derailment.
Governor Andy Beshear declared a state of emergency following the accident on Wednesday afternoon near the town of Livingston.
"Response efforts for the incident are ongoing, and local officials are encouraging those in the town of Livingston to evacuate," the governor's office said in a statement, urging local residents to stay clear of the area.
Livingston resident Cindy Bradley was preparing to celebrate Thanksgiving, a major American holiday during which families get together around a festive meal, when authorities told her to evacuate because "there's 12 to 14 cars in the river."
"We said, 'What, what, what about Thanksgiving?'" she told a local CBS affiliate from a gym in the nearby town of Mount Vernon, which had been turned into a shelter for evacuees.
Bradley added: "It is scary. Unpredictable. We don't know how long we are going to be here, what is going to happen. Our homes are empty. Holidays. It makes you want to cry."
Linda Todd, another Livingston resident at the shelter, lamented that her Thanksgiving dinner preparations had been ruined.
"I was freaking out, because we are cooking, we've got turkeys in the oven. We can't leave," she said.
The emergency order allows the state to activate additional resources to deal with the accident.
"By issuing a state of emergency, we are ensuring that every state resource is available to help keep our families safe," Beshear said in a statement.
"Please stay clear of this area as state, local and CSX officials respond."
The crash comes amid increased scrutiny of rail freight carriers after a train derailed in the midwestern state of Ohio earlier this spring, spilling more than one million gallons of toxic chemicals, including known carcinogen vinyl chloride, into the air, soil and waterways.
More than 2,000 people were forced from their homes in that crash. Train derailments are common in the United States, with more than 1,160 such accidents in 2022 -- or an average of more than three per day -- according to the Federal Railroad Administration.
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