The flaws were noted as far back as a 1991 safety study.
An Associated Press analysis of 20 years' worth of federal rail accident data found that ethanol tankers have been breached in at least 40 serious accidents since 2000. In the previous decade, there were just two breaches.
The number of severe crashes is small considering the total mileage covered by the many tankers in service, and the rail industry's safety record on hazmat shipments is strong. More than 99 percent of hazmat rail shipments arrive safely at their destinations.
"Safety is the freight railroad industry's No. 1 priority," said Patricia Reilly, senior vice president of communications at the Association of American Railroads. She said freight railroads work with experts and federal regulators to develop "rigorous standards for hazmat tank cars."
"Rail continues to be the safest way to transport chemicals," Reilly added.
But the accident reports show that since 1996 at least two people have been killed by balls of flame, with dozens more hurt. And the risk of greater losses looms large.
The rail and chemical industries and tanker manufacturers have voluntarily committed to safety changes for cars built after October 2011 to transport ethanol and crude oil. The improvements include thicker tank shells and shields on the ends of tanks to prevent punctures.
Under the industry proposal to regulators, the 30,000 to 45,000 existing ethanol tankers would remain unchanged, including many cars that have only recently begun their decades-long service lives.