Abstract: Each prefabricated side wall of a transportation type vehicle has a joint surface formed along the lower, inner side thereof for seating thereto the laterally outward edge portions of two flanges which extend laterally outwardly in vertically spaced, parallel relation along each side of a prefabricated floor slab of such vehicle. An upwardly tapered bead along the outer edge of the uppermost of each pair of such flanges enters a recess provided therefor along its respective side wall surface. Also, one half of a plastic-key-interlock joint is formed along the outer edge of the lowermost flange of each pair for interlocking engagement with a second half of each such joint which is formed along each such joint surface. With the side walls substantially perpendicular to the floor slab, upward movement of each tapered bead into its recess also moves the joint halves associated therewith into interlocking engagement with each other, and simultaneously opens up a key-forming passage lengthwise within each joint.