Abstract: A pair of rails mounted on top of the sidewalls of a pickup truck bed, with each rail forming a channel with a slot at the top, to receive mounting feet of accessories. One accessory is in the form of three cover sections with mounting feet received in the channels, and with cover sheet portions that extend beyond the slots and down to at least the rail height to prevent the entrance of water into the truck bed. The rearmost section has a depending flange which lies below the level of the tailgate, so that when the tailgate is locked up, the accessories cannot be removed. When the sections are locked in their adjoining relationship, a U-shaped elastomeric member and a trough enable a complient, elastic seal between adjoining sections. The cover sections may have sidewalls of substantial height to allow use of the truck as a van or camper.