Widely used for temporary gate ways or even permanent ones that are not used often enough to warrant spending on a complete new gate and posts. Usually fitted with square sawn rails but 1/2 rounds can be used if small enough or shaped to fit the brackets. 
Locking arms on the bracket can securly lock the rail into place by cutting a small notch in the rail as shown in the photographs.
This items consists of four brackets, two rail slide bracket and two rail slide locking bracket. This allows two rails to be installed.