The agency says that from 1980 to 2017 there were only 14 accidents involving damage to DOT-113 tank cars, including two where both the outer and inner tank walls were punctured. One accident released ethylene, which burned, and the other involved a release of liquid argon, which is not flammable.
"No injuries or fatalities were reported as a result of the release of hazardous materials from either incident," PHMSA said.
There were four other instances in which damage or failure to the valves or fittings on a DOT-113 caused its liquid cargo to escape.
The LNG tanker cars would be fitted with several pressure-relief systems that allow the contents to escape in the event the liquid expands dramatically, which can happen if the tanks are engulfed in flames in an accident. The relief valves reduce the chances that fuel expands and causes the tank to burst dramatically, triggering a powerful blast known as a BLEVE - a boiling liquid expanding vapor explosion.
"LNG transportation has a good safety record, with minimal maritime, facility, and motor carrier incidents relative to other flammable liquids," a consultant, Cambridge Systematics, said in a report to the agency. "In other countries, LNG has been transported safely by rail with no incidents to date."
But the National Transportation Safety Board (NTSB) was not so sanguine. It said that only 405 DOT-113 tank cars are in service in North America - including 67 of the type that PHMSA was considering for transporting LNG. Given the small sample size, the safety board said it was not a compelling safety record.
In January, the safety agency urged PHMSA to conduct a comprehensive review of the DOT-113's puncture resistance and its ability to withstand fires in accidents before allowing a widespread rollout of the LNG railcars, citing the string of "fiery flammable liquids accidents" that occurred with ethanol and crude-oil trains between 2009 and 2015 until stricter regulations were adopted.
NTSB also urged the transportation agency to require enhanced brakes and reduced speed limits for LNG trains. And to protect train crews in case of an accident, the safety board called for adding a buffer of at least five freight cars between a train's engine and any tanker cars carrying LNG.
"We believe the risks of catastrophic LNG releases in accidents is too great not to have operational controls in place before large blocks of tank cars and unit trains proliferate," Robert Sumwalt, the safety board's chairman, wrote in the group's formal comment to the rules.
PHMSA's final rule called for thicker steel in the outer tanks of the railcars and remote monitoring of the pressure and location of LNG tank cars, but it requires no buffer cars or enhanced brakes. The rule requires railroads to employ multiple locomotives in the middle or at the end of trains that contain larger numbers of LNG tank cars, a practice know as distributed power, which enhances braking.