42 includes a driven roller 73 mounted on and adapted to rotate with driven roller axle 76 and an idler roller 74 rotatably mounted on and adapted to rotate about idler roller axle 77 the bearings 93 may be roller bearings or ball bearings of standard design the driven roller axle 76 and idler roller axle 77 are spacedly mounted at opposite ends of tractor body 75 which may be comprised of side panels in which axles 76 and 77 are rotatably mounted each lower reject tractor 42a includes a driven roller 73a which is mounted on lower drive axle 79 and an idler roller 74a which is mounted on idler roller axle 77a axles 79 and 77a are affixed in a stationary position to frame 35 a flexible continuous belt 43 made of rubber leather or other material with a suitable coefficient of friction is mounted on each tractor 42 42a to extend around each pair of driven roller and corresponding idler roller in the embodiment shown the driven rollers 73 73a and the idler rollers 74 74a of the tractors comprise grooved pulleys and the belts 43 have teeth on their inner driven surfaces which engage the pulley grooves to provide a positive non slip belt drive such pulleys and positive drive belts are well known in the industry the driven roller axles 76 of the upper tractors 42 are coaxially connected to an upper drive axle 78 which rotatingly drives the roller axles 76 to move the belts 43 on the tractors likewise the driven rollers 73a of the lower tractors 42a are driven by lower drive axle 79 to move.