Top rails
As the name suggests, top rails in the fence world refer to a horizontal rail that is installed at the top of a fence.
While many types of fence use top rails, the most common type in North America is probably chain link fence, which uses top rails, top rail sleeves, rail ends, and brace bands to construct the top of the chain link fence structure. Often, residential chain link fences will use swedged top rails, which means that one end of the tube has been narrowed to fit into the end of the adjacent top rail, but this is usually not done on commercial or industrial chain link fences.
Mesh is usually attached to the top rail as well as the fence posts, so the top rail is an integral part of the fence structure.
Top rails can be made of many different types of material, depending on the fence, but when you're talking about chain link fence top rails, you're usually going to be using either a 1 3/8" or 1 5/8" OD tubing or pipe, and that's the size most chain link fittings are designed for.