Abstract: An improved apparatus for covering at least a part of the open bed of a land or marine vehicle or the like includes an enclosure structure telescopically collapsible and extendable with an access opening and door assembly rearward longitudinally movable portion thereof. An improved end gate assembly is selectively positionable and releasably securable at a number of continuously variable longitudinal locations on the open bed, corresponding with the movable rearward section of the enclosure. The apparatus also includes an improved track assembly for the telescopically collapsible and extendable enclosure sections, with apparatus for substantially preventing or minimizing the accumulation of corrosion, water or other foreign materials that would hinder the smooth and free operation of the enclosure structure.