BILLINGS, Mont. (AP) — Attorneys general for North Dakota and Montana asked the Trump administration on Wednesday to overrule a Washington state law that imposed new restrictions on oil trains from the Northern Plains to guard against explosive derailments.

In a legal petition to the U.S. Department of Transportation, Montana Attorney General Tim Fox and North Dakota’s Wayne Stenehjem said federal authority over railroads pre-empts the state law.

Washington Gov. Jay Inslee, a Democrat, in May signed the measure that requires oil shipped by rail through the state to have more volatile gases removed to reduce the risk of explosive and potentially deadly derailments.

The move followed a string of fiery and explosive oil train derailments over the past decade, including a 2013 accident in Lac-Megantic, Quebec, that killed 47 people. The explosions drew widespread public attention to the volatile nature of Bakken crude shipments.

But opponents say the new restrictions would make Pacific Northwest refineries effectively off-limits to crude from the Bakken region, one of the nation’s most productive oil fields straddling the North Dakota-Montana border. That’s because the process of treating the oil to make it less volatile would be too expensive to justify, they said.

“It’s pretty clear in this the state of Washington overstepped its bounds,” Fox said. “The effect would be terrible, both on the economies of North Dakota and Montana and also how it offends the rule of law.”

Fox and Stenehjem also warned that allowing Washington’s law to stand could inadvertently undermine safety, by subjecting the railroad industry to a hodge-podge of state laws instead of a common federal standard.