28 within the rail channel 40 the tray holder 44 preferably comprises a first runner element 46 projecting within the rail channel 40 from the first side wall 36 and a second runner element 48 projecting within the rail channel 40 from the second side wall 38 the runner elements are adapted to slidably receive and support a tray element 28 within the rail channel 40 and further function to restrain movement of the tray element 28 away from the platform 20 towards the mouth 50 of the rail channel 40 each of the runner elements 46 and 48 preferably each comprises an elongate flange having a l shaped cross section each such flange preferably extends continuously and linearly along substantially the entire length of the channel element 26 alternately either or both of the runner elements 46 and 48 may comprise a series of spaced collinear flanges rather than a single elongate flange the first and second runner elements 46 and 48 should be equidistant from the platform 20 with continued reference to fig3 the tray holder 44 preferably further comprises a first stop element 52 projecting within the rail channel 40 from the first side wall 36 and a second stop element 54 projecting.