Local responders, crews from CSX Corp. and cleanup workers were trying to determine how much of the liquid spilled in Willard, about 65 miles southwest of Cleveland.
Four cars derailed at the rail yard late Tuesday while switching trains, and officials believe that's when one car was damaged, though that hasn't been confirmed, said Gary Sease, a spokesman for the freight railroad. He said the nearly full tanker that leaked was carrying about 26,000 gallons of styrene monomer, which is used to make various plastic and rubber products.
The liquid leaked from a 4-inch hole for several hours before it was resealed, Willard's city manager, Brian Humphress, told The Sandusky Register.
Officials warned that the cleanup could take a while, possibly into Thanksgiving Day, and it wasn't clear when evacuated residents might be allowed to return.
The cause of the derailment also was unclear, though Sease said an investigation into that was underway as cleanup continued.
Residents were told overnight to evacuate an area encompassing an estimated 400 homes. They sought shelter with friends and family, at the high school or in local motels.
Jennifer Barnett told The Blade in Toledo that police awoke her family around 2:30 a.m., and she, her husband and her three children were taken to the school by van, without their own vehicle.
"Most likely we're not going to have Thanksgiving with our family," she said.