Ten months ago, government safety officials warned that more than half of the nation's 60,000 pressurized rail tank cars did not meet industry standards, and they raised questions about the safety of the rest of the fleet as well.
Their worry, that the steel tanks could rupture too easily in an accident, proved prophetic.
On Thursday, a derailment in South Carolina caused a catastrophic release of chlorine: eight people died, 58 were hospitalized and hundreds more sought treatment. Thousands of people within a mile of the accident were driven from their homes.
And last summer, a derailment in Texas caused a steel tanker to break open, spewing clouds of poisonous chlorine gas that killed three people.
The causes of the accidents are still under investigation. But the devastation they have wrought shows why tank cars have become an increasing concern not just to safety investigators but also to domestic security officials worried that terrorists could turn tankers into lethal weapons.
The FBI warned in 2002 that al-Qaida might be planning to attack trains in the United States, possibly causing derailments or blowing up tank cars laden with hazardous materials. And after bombings on commuter trains killed 191 people in Spain in March, domestic U.S. security officials secretly persuaded one railroad to reroute toxic shipments that had routinely passed within four blocks of the Capitol in Washington, government officials said.
Federal authorities have been working with railroads and the chemical industry to improve security for trains. But there is still much to be done, particularly given the structural weaknesses of many tank cars, current and former federal officials say.
Just how ruptured tank cars can endanger a community was underscored three years ago when a Canadian Pacific Railway freight train derailed just outside Minot, N.D. Five tank cars carrying a liquefied type of ammonia gas broke open, releasing toxic fumes that killed one resident and injured more than 300.
The National Transportation Safety Board, in a report on the accident released last year, said that the steel shells on the five ruptured tank cars had become brittle, causing a "catastrophic fracture" that released clouds of toxic vapors. Those cars, the safety board found, were built before 1989 using steel that did not - as it does now - undergo a special heat treatment to make it stronger and less brittle. Tank cars built after 1989 use this specially treated steel.
The safety board warned that of the 60,000 pressurized tank cars in operation, more than half were older cars that were not built according to current industry standards, leaving them susceptible to rupture. And because these cars may remain in service for up to 50 years, some older ones could still be hauling hazardous materials until 2039.
Among the hazardous materials carried by the tank cars are liquefied ammonia, chlorine, propane and vinyl chloride. In most cases, chemical or leasing companies own the cars, not the railroads.
"We are required to carry this stuff," said Kathryn Blackwell, a spokeswoman for Union Pacific, the nation's biggest railroad. "We'd rather not, in many cases, but this is one of the things we would like chemical companies to be responsible for."
Although the rail industry now requires that tank shells be made with the special heat-treated steel, the safety board said that treatment alone "does not guarantee" enough protection against impact. Other manufacturing techniques should also be explored, the board said, but it cautioned that the industry and the Federal Railroad Administration "have not established adequate testing standards to measure the impact resistance for steels and other materials used in the construction of pressure tank cars."
Steven W. Kulm, a spokesman for the railroad administration, said, "We have a long history of activities and actions that have improved the integrity of tank car construction." Kulm said that since 1994, accidents "have been few in number," though even one death, he added, was too many. "Tank cars are more crashworthy and puncture-resistant in train derailments today than ever before," he said.
In the Texas crash last summer, the tank car that ruptured and released poisonous gas was made before 1989, though federal investigators have not concluded that brittle steel played a role in that accident.
The South Carolina crash involved the rupture of a newer tank car manufactured in 1993, said Richard Koch, vice president for public affairs at the Olin Corp., a diversified manufacturing company that owned the car.
Koch said that tanker had been recertified to carry hazardous materials in June.
Railroad and chemical executives formed a task force to study the safety board recommendations, and it has been conducting crash tests on about a dozen tank cars.
Michael E. Lyden, the vice president for storage and transport at the Chlorine Institute, a trade group in Arlington, Va., said the leading companies were "working in a cooperative manner to improve the pressure vessels."