Two environmental groups are asking the U.S. Department of Transportation to immediately ban shipments of volatile crude oil in older railroad tank cars, citing recent explosive oil train wrecks and the department’s own findings that those accidents pose an “imminent hazard.” The petition filed Tuesday by the Sierra Club and ForestEthics seeks an emergency order within 30 days to prohibit crude from the Northern Plains’ Bakken region and elsewhere from being carried in the older tank cars, known as DOT-111s.