Federal regulators say rail cars being used to transport oil from North Dakota’s Bakken shale pose an “unacceptable safety risk.” The cars were involved in recent explosions, including one in Quebec that killed 47 people in July. The remarks came a day after regulators issued an emergency order for stricter testing to measure how explosive oil is before it is transferred. Recent tests found more than half of oil samples bound for transfer by train from the Bakken region were misclassified. The region is fueling a boom in U.S. oil and gas, but tests indicate the oil there is more explosive than other types.