Abstract: A conveyor belt structure (10) has a pair of longitudinally extending tracks or stringers (11) upon which trolleys can ride. A power pack trolley (12) also rides stringers (11). The trolley (12) has stringer-engaging wheels (14) and a power pack supported upon a frame. The power pack might be a rechargeable battery or a generator for example. The power pack trolley (12) also includes a motor or motors to drive its wheels (14). A captivation (30) is affixed to a bogie arm (31). Attached to the lower edge of the mounting plate (32) is a horizontal plate (34). A flange (36) is angularly offset with respect to the plane of the plate (35). Should one of the wheels (17) of the trolley (16) become derailed from stringer (11), one or both of plates (34 and 35) of the captivation bracket (30) will strike the stringer (11).