The Speed restrictions had been put in place by track owner CSX on July 22 because of unfinished track repairs. But thinking the work had completed, the supervisor lifted the speed limit several days later. When the Capitol Limited, pulling 13 passenger cars from Chicago to Washington, reached that stretch of track July 29, it should have been following the limit, according to the NTSP report.

Yesterday's update offers new clues into the accident, which injured 101 of the 176 people on board minutes before they would have reached their destination. Until now, the investigation appeared to focus on heat-related-related problems. The train's engineere reported seeing misshapen rail before the derailment, and investigators later discovered small kinks in nearby sections of track.

"Whether lifting the slow order, the summer heat or any other or combination of factors contributed to[the derailment]is obviously part of the continuing investigation," said CSX spokesman Gary Sease. He decline to comment on whether the track supervisor had been disciplined.