An upper rail supporting plate is interconnected by resilient material to a lower foundation plate for lateral and vertical movement. Lateral movement of the upper plate transverse to the rail direction is limited by reinforced upstanding flanges upon a first pair of edges of the lower plate. The reinforcement includes horizontal flanges projecting outwardly from the upper edges of the upstanding flanges. Additional flanges upon a second pair of lower plate edges contribute further reinforcement, and limit upper plate movement in the rail direction. Toothed washers mount the lower plate for lateral adjustive movement in preselected increments of differing magnitudes.