A barrier is formed from a plurality of posts and a plurality of rails. Each post has an I-shaped cross section, with a pair of double-walled flanges separated by a single-walled web. Each rail is a channel-shaped member having an intermediate portion and opposed end portions. Each end portion has a rounded convex edge. The intermediate portion includes a flat section to which pickets or a fencing fabric may be attached. Each end portion also includes a flat section, which is recessed relative to the flat section of the intermediate portion. A rail is connected to a post by inserting the rail's end portion into the channel-shaped region of the post. The flat section of the rail's end portion engages the planar inner side of one of the post's flanges. Fasteners are inserted into aligned openings in the flange and the flat section to connect the rail to the post. When assembled, the post flanges and the flat section of the intermediate portion of the rail are coplanar.