A subway train derailment that injured nearly three dozen people and sparked major delays is being blamed on human error, not a track defect, and two supervisors have been suspended while the matter is investigated.

The supervisors were responsible for oversight of work being done on the tracks, MTA spokesman Kevin Ortiz said Wednesday. “They have been suspended without pay pending a formal review process,” he said.

A preliminary investigation indicates the derailment Tuesday morning in Harlem was caused by “an improperly secured piece of replacement rail that was stored on the tracks,” Metropolitan Transportation Authority Chairman Joe Lhota and Interim Executive Director Ronnie Hakim said in a joint statement late Tuesday.