can vary be it horizontal or inclined the one side surface 17 of the flexible slab 10 is a generally convex shape paralleling that of the contour of the rail such as to merge with the bottom portion of the slab that follows the contour of the base 4 as seen in fig1 the uppermost portion of such one side surface 17 is generally planar and in abutting contact with the head 2 of the rail 1 the portion of side surface 17 between the planar surface and the convex side portion is recessed as at 18 this construction of the slab 10 provides a compression chamber in recess 18 in cooperation with the head 2 and web 3 the side 20 of slab 10 opposite that of side 17 has an upper planar surface adjacent the top surface 14 and a lower planar surface adjacent the bottom surface of the slab side 20 intermediate the upper and lower planar surfaces of slab 10 is arcuate shaped to provide a recess within which asphalt or other fill material which is used holds the slab 10 in position against the rail the slab 10 adjacent to rail 1 is relatively narrow and an asphalt or other road surface construction material is formed snugly against the outside surface of side 20 thereof so as to form an unbroken.