OMAHA, Neb. (AP) — The number of trains carrying at least a million gallons of volatile crude oil across Nebraska and Iowa has increased significantly this spring.

BNSF railroad has told emergency management officials in both states that it is now hauling 20 to 30 trains loaded with oil from North Dakota’s Bakken region through the area.

That’s up significantly from last summer when federal regulators began requiring railroads to notify state officials about trains carrying at least 1 million gallons of oil.

Railroad shipments of crude oil are facing additional scrutiny and tougher regulations because there have been several fiery derailments involving the commodity in recent years. The worst one happened in July 2013 and killed 47 people in a small Canadian city just across the U.S.-Canada border from Maine.