The A Train derailment in Harlem on Tuesday was caused by an unsecured piece of rail line that was left on the tracks.

The MTA said Tuesday night that the cause appeared to be human error and not a defect as was originally believed.

The agency added in a statement that equipment is commonly stored in between the tracks.

However, proper procedure is to have the equipment bolted down — which was not the case in the instance.

The misplaced equipment cause an A train to derail early Tuesday, causing scores of injuries.