Abstract: An apparatus for mounting relatively small and deformable elongated rails on a relatively large, generally horizontal, and nondeformable elongated rail beam has a stiff but limitedly elastically deformable elongated jig. A plurality of clamps spaced longitudinally along the jig are engageable with the rail for locking onto the rail at respective longitudinally spaced locations. Respective vertical and horizontal jacks transversely engage between the clamps and the jig for elastically deforming the jig into a desired shape relative to the rail beam. Holders and the like are provided on the jig for securing the deformable rails to the rail beam in predetermined positions relative to the deformed jig. If the rail section is to be curved in any direction the jig is appropriately deformed relative to the rail beam so its various holders and mounting fixtures lie on the appropriate curve. Such elastic deformation of the jig therefore perfectly positions these rail holders.