EMPLOYEE APPROACHED LOCOMOTIVE TO BOARD IT AS IT WAS MOVING, WHICH IS NORMAL. HE GRABBED THE HAND HOLDS AND PLACED LEFT FOOT ON STEP TREAD. HIS LEFT FOOT SLIPPED ON ICE ON THE TREAD, SLID TO T HE LEFT, LOST BALANCE AND FELL TO HIS RIGHT KNEE ON THE TREAD, CUTTING HIS RIGHT KNEE. HE HAD TO RELEASE HIS GRIP AND FALL AWAY FROM THE TRAIN.