CSX Transportation has determined that the cause of the Aug. 5 derailment in the Howard Street Tunnel was defective track, spokesman Gary Saese said Thursday.

Saese said "there's no doubt" that a broken rail caused 13 cars to jump the tracks in and near the more than 100-year-old tunnel — the site of a more serious derailment in 2001 that led to a chemical fire that disrupted downtown Baltimore for a week.

The spokesman said the railroad had not determined what led to the break, but he said such damage typically is caused by an internal defect.

Saese said the track in the tunnel had been scheduled for replacement before the derailment. He said work on the project would begin this month and be finished around Labor Day.

—Michael Dresser