NEARLY 2,000 residents of eastern Ohio remained under evacuation orders on Monday as railroad crews drained and burned off a toxic chemical from five tanker cars of a freight train that derailed in a fiery wreck three days earlier, officials said.

The venting of pressurized 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, a spokesperson for the Ohio Emergency Management Agency.

"That controlled release was the one explosion," she told Reuters by telephone. "It went as planned. It seemed to be a successful incident."

No injuries were reported, either from Monday's operation or the accident on Friday night, authorities said.

Live video on Monday showed a towering column of thick, black smoke rising from the accident site in East Palestine, Ohio, a town close to the Pennsylvania border northwest of Pittsburgh.

The train, operated by Norfolk Southern Railroad and consisting of three locomotives and 150 freight cars, was headed from Illinois to Pennsylvania when it derailed shortly before 9 p.m. EST on Friday, setting off a massive fire that forced the evacuation of hundreds of homes in the immediate vicinity.

About 50 cars actually left the tracks, 20 of which carried hazardous materials, according to the National Transportation Safety Board (NTSB).

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."

Ohio Governor Mike DeWine said in a statement the chemical contents of the five rail cars in question were "unstable and could potentially explode, causing deadly disbursement of shrapnel and toxic fumes."

Working with state and local emergency officials, Norfolk Southern said on Monday 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, DeWine and Pennsylvania Governor Josh Shapiro ordered evacuations expanded on Monday to encompass all homes within a 1- to 2-mile area around the derailment site on both sides of the state line.

Peggy Clark, a spokesperson for the Columbiana County Emergency Management Agency, said the mandatory evacuation covered an estimated 1,900 people on the Ohio side alone.

DeWine's office warned that 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 colorless, industrially produced gas that burns easily and is used primarily in the manufacture of polyvinyl chloride (PVC) pipe and other products, according to the National Cancer Institute. It also is a byproduct of cigarette smoke.

The precise means by which crews vented the gas was not explained. But the railroad said workers had prepared drainage pits and embankments, apparently to contain residue from the release. State environmental officials monitored air quality, it said.

Nearly two hours after the operation began, the company said the "controlled breach" had been "completed successfully."

The cause of the derailment was under investigation by the NTSB, but 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."