A system for fastening a rail to a substrate has an anchor in the substrate, a fastener having a shank secured in the anchor and a head above the substrate, an angle guide plate vertically traversed by the shank, an annular part between the head of the fastener and the guide plate, and a spring clip engaged between the head and the part and shiftable transversely of the rail and of the shank between a preinstallation position horizontally offset from and not engaging the rail and a final installation position bearing vertically downward on the rail. A seal ring engages in both the preinstallation position and the final installation position around the shank and compressed between the part, the angle plate, and the shank so as to prevent water from traveling down the shank into the anchor.