A rail-mounting assembly has a generally rectangular base frame formed with a window and having a pair of diagonally opposite abutments at respective ends of the window and having downwardly and inwardly inclined faces. A top plate in the window has a lower face above a lower face of the frame, a pair of bosses projecting upward from the top plate and each juxtaposed with a respective one of the abutments and each having a downwardly and inwardly inclined face spacedly juxtaposed with the downwardly and inwardly inclined face of the respective abutment. A one-piece elastomeric body is bonded to and interposed between the inclined faces of the frame and of the top plate.