to high laterally directed compressive loads in service composite insert 40 has a compression fatigue lifetime that is substantially longer than that of the body material of improved insulator spacer 24 that contains composite insert 40 composite insert 40 is designed to place a wear resistant durably tough material in contact with the adjacent surfaces of the rail 4 and the shoulder insert 8 10 thereby enhancing the mechanical lifetime of the improved insulator spacer 24 of which it is a part a high compressive strength electrically insulating material is sandwiched between the outer layers of composite insert 40 to provide electrical insulation between the rail 4 and the shoulder insert 8 10 fig5 shows an embodiment of a composite insert 40 having three layers in this embodiment composite insert 40 comprises first outer layer 42 insulating layer 44 and second outer layer 46 in service first outer face 48 of first outer layer 42 and second outer face 50 of second outer layer 46 are in contact with respectively the vertical face of the rail base 16 and the rail facing surface of the shoulder insert 8 10 so that composite insert 40 carries most of the laterally directed compressive load that is exerted in service upon improved insulator spacer 24 by the rail 4 and the.