ARCHIVED - Preventing the domino effect on tank car derailments

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August 01, 2011— Ottawa, Ontario

About 50 years ago, a cylindrical railway tank car that derailed would often ride up until it de-coupled. Once freed, the tank car’s protruding couplers often punched holes in the ends of adjacent cars as they folded into each other like a giant accordion. The ruptured cars would then spill their contents on nearby ground or into waterways, causing tremendous environmental damage if they carried toxic goods. 

To deal with this, three decades ago Canada’s transport ministry ruled that tank cars needed new safety equipment: reinforced shields on the ends of tank cars, and new couplings with “shelves” — extra fittings that barred them from riding up and unhitching. 

“A normal coupler doesn’t have any shelves on it, so if one car goes up or down, the couplers can disengage from each other. They just slide up or down,” says Philip Marsh, manager of test and evaluation at the NRC Centre for Surface Transportation Technology (CSTT) in Ottawa. 

But even as this fix improved railway environmental safety statistics, it became a lesson in unintended consequences. A new, unforeseen problem arose: “domino derailments.”   

Double shelf couplers stopped derailed tank cars from puncturing each other because the cars now locked to each other in both the vertical and horizontal planes. But if one car derailed and tipped, its new coupler was so stiff that the car could now drag and roll over dozens more down with it, like so many heavy metal dominoes. This effect was greatest on trains consisting of near-empty tank cars travelling at low speeds.   

“The issue with that was that a simple one-car tip-over became a huge 40- or 50-car derailment, which is much more costly,” says Marsh.

Pushing the first domino

Pushing the first domino

Several such derailments have occurred across North America in the last decade alone. Faced with a slowly growing list of domino derailments, some involving dangerous goods cars, Transport Canada asked the CSTT for research help to address this problem — possibly using some combination of two existing coupler designs. 

Tipping over tank cars 

It took meticulous preparation. Beginning in 2008, a literature review gave the team enough information to start modelling domino derailment experiments in early 2010. These models helped them plan for full-scale physical testing. By the winter of 2011, they felt ready to start tipping actual tank cars.   

CSTT engineer Albert Wahba says the NRC team spent much of its preparation time calculating how to keep observers safe and protected while it derailed full-sized train cars, and where exactly to place instruments to gauge the many forces and torques that came into play. 

Since the dominoes in question weighed more than 30 tonnes each, the team also needed a 110-tonne-capacity crane to start them falling. 

The derailment experiment proposed and tested a combination of technologies that already existed in other areas of the rail industry. One approach involved welding double shelves onto a rotating coupler used on coal and grain hopper cars. Such cars are designed to be emptied by purposely tipping them and pouring out their contents, one by one, while they’re coupled into a train. 

“The added rotation allows the coupler to rotate with respect to the car. Then the next car won’t be pulled over in the domino effect,” says Marsh. “We tested that approach here successfully. We just took the existing rotating coupler design and added the shelves.” 

“We’re getting the advantages of both the rotary coupler and the double-shelf coupler in one new coupler,” says Wahba. He adds that CSTT’s research and technology development will allow Transport Canada to better understand tank car domino rollover derailments and to assess potential solutions that could reduce derailments when trains are transporting dangerous goods.

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National Research Council of Canada

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