"In an investigation of this sort, we try to look at everything, but at this point, the speed was where it was supposed to be coming into the curve, and there were no apparent anomalies that the crew noticed coming up on that curve," NTSB member Earl Weener said late Wednesday.

The track had been visually inspected Monday, as it is twice a week, and had undergone more thorough testing in April, one of three such inspections each year, Weener said.

The 98-car-freight train was traveling from Chicago to Linwood, N.C., Norfolk Southern spokesman Dave Pidgeon said. In all, 16 cars went off the tracks.

Nobody aboard the train was hurt, but two people suffered minor injuries while walking along the tracks to investigate when an explosion occurred, authorities said.

Columbus Mayor Michael Coleman and public safety officials said they were grateful the accident occurred in the middle of the night, away from more populated areas. About 100 residents within a 1-mile radius were evacuated but were able to return later Wednesday.