realized that the positions of the plain wheels 66 and the flanged wheels 68 are alternated by varying the orientation of the trolleys 50 so that along a particular track or rail 30 there will be first the flanged wheels 68 from one trolley then the plain wheels 66 on the next trolley followed by the flanged wheels on the third trolley and so on this construction is desirable in achieving a type of folding action in which the panels 52 are stored next to one another in a side by side relationship on either of the terminal sections 16 or 18 on the end sections 22 or the end sections 26 with the present invention small cams 70 and 72 are located on the first connecting track 44 and on the second connecting track 46 respectively in each junction section 20 these cams 70 and 72 are spaced from the tracks 44 and 46 slightly as shown so as to accomodate movement of the flanged wheels 68 but are positioned so as to engage the flanges 74 on these wheels 68 in order to be capable of directing the trolleys 50 thus as a trolley 50 is moved so that the flanges 74 on its wheels 68 engage a cam 70 such engagement will tend to cause rotation of the bolt 60 relative to the trolley 50 causing this trolley 50 to move generally in the direction of the connecting track or rail 44 similar engagement of a flanged wheel 68 with a cam 72 will cause a trolley 50 to move along and in the direction a second connecting track or rail 46 because the action achieved with.