Norfolk Southern believes a software defect — not a hacker — was the cause of the widespread computer outage that forced the railroad to park all of its trains for most of Monday. The railroad said Friday that it traced the problem to a defect in the software one of its vendors was using to perform maintenance on its data storage systems. Both the railroad’s primary and backup systems became unresponsive at the same time. The Atlanta-based railroad reiterated that it hasn’t found any evidence of hacking. Regulators have been scrutinizing Norfolk Southern’s operations ever since one of its trains derailed in East Palestine, Ohio in February and an assortment of toxic chemicals caught fire.