The earth underneath railway tracks gave way early Monday, derailing several freight cars and spilling an unknown amount of coal into the Yellowstone River, authorities said.

No one was reported hurt in the train accident, and the coal was not believed to pose a major risk to humans.

Nine of the train's 123 coal cars and its trailing locomotive derailed in a rural area at about 1:15 a.m. Monday, according to Gus Melonas, a spokesman for Burlington Northern Sante Fe Railway. The incident left at least one coal car and the locomotive partially submerged in the river, which runs next to the tracks less than a quarter mile from Interstate 94.

The railroad had cleanup and construction crews on the scene, Melonas said, adding that the derailment on Burlington Northern's line would not delay any passenger travel.

The derailment occurred because the river's "significantly" high water level undermined the railway bed, according to Melonas. About 125 feet of track collapsed and washed away under the weight of the train.

The accident happened about 4 miles west of Forsyth, a town of nearly 2,000 residents that's about 100 miles northeast of Billings. A dispatcher for the Rosebud County Sheriff's Office confirmed the derailment but could not provide details on the incident.

The train originated in Decker, Mont., and was headed to Superior, Wisc.