The Transportation Department issued an emergency order Tuesday requiring stricter standards for shipping oil by rail, a move that follows a series of derailments and growing concerns that crude from the Bakken shale formation is more flammable than most.

The order, which is effective immediately, mandates greater testing of crude oil from the Bakken to ensure it’s properly classified before being loaded onto rail cars. It also calls on those who pack crude oil onto rail cars to adhere to the strongest two federal levels, which require a “more robust tank car,” according to the DOT.