GALENA, Ill. (AP) - The rail cars that split open and burst into flames during a western Illinois oil train derailment this week had been retrofitted with protective shields to meet a higher safety standard than federal law requires, according to railroad officials.

The fire continued to burn Friday, a day after the derailment in a rural area south of the city of Galena. No injuries were reported, but the accident was the latest in a series of failures for the safer tank-car model that has led to some people calling for even tougher requirements.

"It certainly begs that question when ... those standards failed to prevent leakage and explosions that threaten human safety and environmental contamination," said Steve Barg, director of the Jo Daviess Conservation Foundation, which owns a nature preserve several hundred yards from the derailment site.

BNSF Railway said the train's tank cars were a newer model known as the 1232. It was designed during safety upgrades voluntarily adopted by the industry four years ago in hopes of keeping cars from rupturing during derailments.

But 1232 standard cars have split open in three other accidents in the past year, including one in West Virginia last month. That train was carrying 3 million gallons of North Dakota crude when it derailed, shooting fireballs into the sky, leaking oil into a waterway and burning down a house. The home's owner was treated for smoke inhalation, but no one else was injured.

In Thursday's accident in Illinois, 21 of the train's 105 cars derailed in an area where the Galena River meets the Mississippi. BNSF Railway said a resulting fire spread to five rail cars. Firefighters could only access the derailment site by a bike path, said Galena Assistant Fire Chief Bob Conley.

Emergency personnel were still working to contain the blaze Friday but described the area as "stable." The Federal Railroad Administration was investigating.

The train had 103 cars loaded with crude oil from the Northern Plains' Bakken region, along with two buffer cars loaded with sand, according to company spokesman Andy Williams. The cause of the derailment hasn't been determined.