Brookville battery powered manbus arrived on #2 section. Operator hit e-stop & knocked power. Bus started to roll & operator did not re-energize machine. Bus traveled down grade for approx 400' where it derailed in a curve & hit rib head-on. Injured ee, a passenger, was transported to CAMC where he received six staples to close a cut to the head. He lost no days due to his injury.