CSX Transportation has issued a probable cause for the December 19 derailment in Marietta, Ohio, that caused the Virginia Street overpass to collapse onto the roadway below.

Neal F. Zimmers, vice president for CSX in Ohio, previously had said the cause of the wreck was an internal fault in the track itself, a problem no larger than the size of a dime. Engineers with CSX said this type of defect is very rare and can only be detected by X-ray, an inspection that is done only once a year.