According to the company, a maintenance crew had been replacing crossties on the bridge.
Company investigators said the crew did not follow engineering protocols, and failed to sufficiently anchor temporary fasteners to the ties.
The oil train derailed on January 20th, leaving some of the cars partially hanging over the Schuylkill River.
The cause finding is subject to official review by the Federal Railroad Administration, CSX said.
The company went on to say the work was not related to the structural integrity of the bridge, which remains sound.
