Emergency crews released toxic chemicals into the air from a freight train that was in danger of exploding, then set it alight after warning residents near the Ohio-Pennsylvania state line to leave immediately or risk death.
Almost 2,000 residents of eastern Ohio remained under evacuation orders because of the dangers of toxic chemicals from five tanker cars of a train that derailed in a fiery wreck three days earlier, officials said.
The venting of pressurised vinyl chloride, a highly flammable and carcinogenic gas, began with a single explosion, as was anticipated, followed by a steady incineration of the remaining cargo, said Sandy Mackey, spokeswoman for the Ohio Emergency Management Agency.
"It went as planned," Ms Mackey told Reuters. "It seemed to be a successful incident."
No injuries were reported, authorities said.
Live video showed a towering column of thick, black smoke rising from the accident site in East Palestine, Ohio, a town close to the Pennsylvania border north-west of Pittsburgh.
The train, operated by Norfolk Southern Railroad and consisting of three locomotives and 150 freight cars, was travelling from Illinois to Pennsylvania when it derailed on Friday, setting off a hfire.
Public safety concerns deepened after the railroad said pressure-relief devices on some tankers were found on Sunday to have stopped working, which the company said could "result in a catastrophic failure".
Working with state and local emergency officials, Norfolk Southern said on Monday that it devised a plan to manually vent the cars, allowing the contents to "be drained in a controlled fashion" under supervision of "experts and first responders".
As part of the plan, Ohio Governor Mike DeWine and Pennsylvania Governor Josh Shapiro ordered the evacuation of all homes within 3.2km of the derailment site, on both sides of the state line.
Mr DeWine's office said fumes released into the air from the venting operation could be deadly if inhaled, while also posing the risk of skin burns and serious lung damage.
Vinyl chloride is a colourless, industrially produced gas that burns easily and is used mainly to make polyvinyl chloride pipe and other products, according to the National Cancer Institute.
It also is a byproduct of cigarette smoke.
Nearly two hours after the operation began, the company said the "controlled breach" had been "completed successfully".
National Transport Safety Board member Michael Graham said on Sunday that video footage of the accident pointed to possible "mechanical issues on one of the rail car axles".
The cause of the derailment is under investigation.
Agencies contributed to this report.
