A train that partly derailed and exploded north of downtown was traveling below the recommended speed before the crash, and its operators noticed no problems with the track as they approached the curve where it happened, a National Transportation Safety Board representative said.

The NTSB is looking into what caused the Wednesday derailment on Norfolk Southern Corp. tracks north of downtown Columbus, which led to spectacular explosions and the burning of three tank cars, each carrying 30,000 gallons of ethanol.

Authorities said they decided to let the tankers burn for a while and then spray chemical foam to prevent the blaze from re-igniting.

The smoldering fire extinguished early Thursday morning, and Norfolk Southern has begun moving cars away from the scene, a company spokesman said.

Investigators are reconstructing the rails in hopes of spotting any problems. The full investigation could take a year.

"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.

The Columbus crash was unusual because it occurred on main line tracks, instead of more common derailment sites such as rail yards or industrial facilities, Federal Railroad Administration spokesman Warren Flatau said.

Of the 389 derailments in the U.S. in the first four months of 2012, 96 were on main lines, Flatau said. There were 22 in Ohio, four of them on main lines in Huron, Lorain, Fairfield and Sandusky counties.

Last year, 419 of the total 1,451 derailments nationwide were on main lines. Ohio had 65 derailments; a dozen were main-line.

Federal Railroad Administration data show Norfolk Southern to have the fewest number of accidents and reported incidents over the past decade of the four major freight carriers that operate on U.S. rails. The company's reported number of train accidents on main lines since 2001 — 695 — is also the fewest of the four major carriers, the statistics show.

Of those major lines, Union Pacific reported 2,019 accidents on main lines during that period, followed by BNSF Railway Co. (1,781), and CSX Corp. (1,005).

Flatau said Norfolk Southern and CSX primarily serve the eastern United States, with BNSF and Union Pacific serving the west.

Roughly 29.4 million carloads of freight are hauled every year across 140,000-plus miles of rail in the United States, according to the American Association of Railroads.