This story is getting some exposure here in Winnipeg. The Transportation Safety Board (TSB) released a report on a tank car incident at Dugald, MB in January 2009.
Basically the end sill (including the coupler) tore off the end of tank car UTLX 37605 when CN 304 was starting to pull. Essentially this is a broken knuckle issue, except that instead of a broken knuckle, a piece of the tank car pulled off. The train went into emergency as expected, there was no derailment and no release of hazardous materials. The issue is that the end sill was known to be damaged and the car should never have been on that train.
The report has some interesting reading. Union Pacific noted the sill was cracked back in mid November. A temporary weld was made and the car was to be handled only as the last car in a train, and not to be humped. When it crossed the border into Canada, that note was lost and the car was handled many times. It was supposed to go to Procor for repair but it appears that instruction was also lost.
This conclusion is particularly damning: "Deficiencies in Canadian National's (CN) waybilling and car tracking systems permitted tank car UTLX 37605 to be placed on 6 different trains, switched at least 13 times and humped 7 times with a severely damaged and cracked A-end stub sill."
I think CN is fortunate that the accident happened at such a low speed and there was no release of product. Hopefully corrective actions have been taken and this type of event will not happen again.