Abstract: A hollow body including spaced apart side walls, with a plurality of longitudinally spaced, laterally extending wall-support members surmounting a plurality of laterally spaced longitudinally extending floor reinforcement members. A track structure including a plurality of laterally disposed track structure support members projecting exteriorly of said walls in longitudinal alignment with, and secured to, said wall-support members. Said track structure further including an upper fixed runway and, substantially parallel thereto, a lower fixed runway, both elongated longitudinally of the vehicle. A supplementary runway surmounting said upper, fixed runway. Means to secure said supplementary runway above said upper, fixed runway. Means urging said supplementary runway toward said upper, fixed runway, and means adjustably resisting said urge, to space said supplementary runway from said upper, fixed runway. An endless chain drive, and means to drive said chain.