"The cars were a pile of metal," Chris Lowie, manager of the federal refuge said.

"They were accordioned, literally. Most of them were sideways, busted up and tipped over. Coal was everywhere."

Norfolk Southern said in a statement the spill "is confined to a relatively small area" adjacent to the tracks and that "there is no impact to any major waterway".

The company said its personnel were on site and coordinating the clean-up with the US Fish and Wildlife Service, the US Army Corps of Engineers and the Virginia Department of Environmental Quality.

The park is home to endangered species such as long-eared bats and red-cockaded woodpeckers. But the spill may most likely impact aquatic species such as snakes and turtles.

There are no public roads for miles, which means clean-up crews are reaching the scene by taking old logging roads.

The Great Dismal Swamp was once an impenetrable morass where explorers vanished and runaway slaves escaped. Before he became the first American president George Washington,  a legislator in Virginia kicked generations of loggers out of there some 200 years before the swamp became a national wildlife refuge in 1974.

Since then, efforts have been underway to restore it. The derailment's impact remains unclear, Lowie said.

AP