as will be understood by one having ordinary skill in the art a top edge 49 fig1 of section 46 is recessed so that a forward edge of the side wall constitutes a flange 52 that overlaps the adjoining edge of side rail section 48 by constructing the side rails 42 in this manner they articulate downward as shown in phantom in fig1 to allow the cart sections to u201c bend u201d when they roll around the arcuate front section 30 of the track 24 when the cart sections 34 are positioned in a horizontal position on the upper track 26 however upper edge 49 on section 46 engages upper edge 51 on section 48 to prevent side rail 46 from pivoting upwardly with respect to side rail 48 beyond the horizontal position shown in fig1 this also prevents the cart 32 from sagging under load when a storage unit or pallet 19 is mounted on the cart the articulating side rails 42 are an important feature of the present invention because of the dimensions of the structure in the preferred practice of the present invention the distance between the upper and lower track sections 26 and 28 respectively is minimized and desirably is about 11 5 inches 292 mm the.