In March, the NTSB asked for the higher standards to be applied to all tankers, meaning existing cars would have to be retrofitted or phased out.
The industry's proposal "ignores the safety risks posed by the current fleet," the NTSB said in a report on safety recommendations, adding that those cars "can almost always be expected to breach in derailments that involve pileups or multiple car-to-car impacts."
The federal Pipeline and Hazardous Materials Safety Administration, part of the U.S. Department of Transportation, is considering both arguments, but the regulatory process is slow and could take several years, experts said.
Industry representatives say a retrofit isn't feasible because of engineering challenges and costs. They insist the threat of serious accidents is overstated.
"How many millions of miles have the 111 cars run without problems?" said Lawrence Bierlein, an attorney for the Association of Hazmat Shippers Inc. "It's more likely you're going to be hit by lightning."
But worries about the tankers' weaknesses persist, especially since the volume of dangerous cargo on American rails is only expected to grow.
Ethanol production has soared from 900 million gallons in 1990 to nearly 14 billion gallons last year. Seeking to lessen America's dependence on foreign oil, federal mandates will quadruple the amount of ethanol and other renewable fuel that's blended into the nation's gasoline and diesel by 2022.
Nearly all of it moves by rail. In 2010, that meant 325,000 carloads of ethanol, according to the Association of American Railroads. Ethanol is now the highest-volume hazardous material shipped by rail. In 2000, it wasn't even in the top 10.