Following a series of explosive accidents, U.S. federal officials said Thursday that crude oil being shipped by rail from the northern plains may be more flammable than traditional forms of oil.A safety alert issued by the U.S. Department of Transportation warns the public, emergency responders, and shippers about the potential high volatility of the crude being shipped from the Bakken oil-shale region in Montana and North Dakota.The warning comes after a massive explosion caused by an oil-train derailment Monday near Casselton, North Dakota. Nobody was hurt, but concerns about toxic fumes prompted the evacuation of hundreds of residents.In July, 47 people were killed in a Quebec, Canada town when a train carrying Bakken crude derailed. Another oil train derailed and exploded in Alabama in November, killing nobody but spilling 2.85 million liters of oil.The amount of oil moved by rail has spiked since 2009, from about 10,000 tanker cars to a projected 400,000 in 2013