Abstract: A two piece top rail assembly for interchangeable use in constructing both open top trailers with exterior posts and exterior skin. The rail assembly includes an elongated outside element having in cross section a depending base leg for securing to the posts on the trailer. A lower cross leg is secured in angle to the base leg and an intermediate leg is secured to the lower cross leg and has at least a portion extending substantially in the opposite direction to the base leg. An upper cross leg is secured to the intermediate leg and extends at an angle substantially in the direction toward the base leg. An inside element that may be used in two positions in contact with the outside element includes a connecting leg extending between and connecting first and second transverse leg. Openings which are in alignment are provided in the first and second transverse leg for receiving the roof bow.