A sealing plate for use with a rail clip anchoring device having a head and a stem which extends from the head into a concrete sleeper when the anchoring device is in use, is adapted for extending over the underside of the head when the stem of the device is being set in a concrete sleeper, thereby to prevent ingress of concrete into the head of the device, and for being retained on the surface of the sleeper thereafter. A major face of the plate, which is uppermost when the sleeper is in use, is provided with at least one clip seat portion for receiving part of a rail clip. In a method of manufacturing a concrete sleeper with at least one embedded rail clip anchoring device, the floor of a mold, from which the sleeper is to be formed, is provided with an aperture through which the head of an anchoring device is inserted and before concrete is introduced into the mold the aperture is sealed off around its edges and around the underside of the head of the anchoring device by placing a sealing plate over the aperture on the floor of the mold, the plate overlapping and sealing around the edges of the aperture other than where the anchoring device is located and having a mating interface with the anchoring device such that a seal is formed therewith.