A cable tray assembly, comprising: - two cable tray segments, each of said two cable tray segments having a bottom wall and two side walls facing one another extending respectively from the two longitudinal sides of said bottom wall, said side walls having an outer side having a C-shaped section with an upper wing and a lower wing joined together by a web; wherein said two cable tray segments are cable ladder segments each of which is formed by two coplanar side rails attached to one another by a plurality of cross members spaced apart from one another, said two side rails constituting said side walls of the cable tray segment, and said plurality of cross members constituting a discontinuous bearing surface for cables which constitutes said bottom wall of the cable tray segment; and wherein the inner side of each of said side rails, facing the opposite side rail, and the two ends of each of said cross members are formed such that said ends of the cross members are press fit onto said inner sides of the side rails. - and a joining plate, made up of a single part, which is coupled to two of said contiguous side walls, of the two contiguously arranged cable tray segments, for joining them together, said joining plate having two longitudinal edges which rest on said upper wing and lower wing of the two contiguous side walls and said joining plate being formed such that it is press fit between said upper wing and said lower wing of the two contiguous side walls; characterized in that: each of said cross members is a profile made of a polymer material and each of said side rails is a profile made of a polymer material; said joining plate is made up of a single part made of a polymer material; said web of the contiguous side walls has an outer face provided with at least one rib extending in the longitudinal direction of said side wall, and said joining plate has a planar face which rests on said outer face of the web of the two contiguous side walls, said planar face comprising at least one groove extending along said joining plate in the longitudinal direction thereof and in which said rib of each of the two contiguous side walls fits when said joining plate is press fit between said upper wing and lower wing of the two contiguous side walls. - two cable tray segments, each of said two cable tray segments having a bottom wall and two side walls facing one another extending respectively from the two longitudinal sides of said bottom wall, said side walls having an outer side having a C-shaped section with an upper wing and a lower wing joined together by a web; wherein said two cable tray segments are cable ladder segments each of which is formed by two coplanar side rails attached to one another by a plurality of cross members spaced apart from one another, said two side rails constituting said side walls of the cable tray segment, and said plurality of cross members constituting a discontinuous bearing surface for cables which constitutes said bottom wall of the cable tray segment; and wherein the inner side of each of said side rails, facing the opposite side rail, and the two ends of each of said cross members are formed such that said ends of the cross members are press fit onto said inner sides of the side rails. - and a joining plate, made up of a single part, which is coupled to two of said contiguous side walls, of the two contiguously arranged cable tray segments, for joining them together, said joining plate having two longitudinal edges which rest on said upper wing and lower wing of the two contiguous side walls and said joining plate being formed such that it is press fit between said upper wing and said lower wing of the two contiguous side walls; characterized in that: each of said cross members is a profile made of a polymer material and each of said side rails is a profile made of a polymer material; said joining plate is made up of a single part made of a polymer material; said web of the contiguous side walls has an outer face provided with at least one rib extending in the longitudinal direction of said side wall, and said joining plate has a planar face which rests on said outer face of the web of the two contiguous side walls, said planar face comprising at least one groove extending along said joining plate in the longitudinal direction thereof and in which said rib of each of the two contiguous side walls fits when said joining plate is press fit between said upper wing and lower wing of the two contiguous side walls. each of said cross members is a profile made of a polymer material and each of said side rails is a profile made of a polymer material; said joining plate is made up of a single part made of a polymer material; said web of the contiguous side walls has an outer face provided with at least one rib extending in the longitudinal direction of said side wall, and said joining plate has a planar face which rests on said outer face of the web of the two contiguous side walls, said planar face comprising at least one groove extending along said joining plate in the longitudinal direction thereof and in which said rib of each of the two contiguous side walls fits when said joining plate is press fit between said upper wing and lower wing of the two contiguous side walls.