A support rail for a movable, flatbed trailer-mounted tarp enclosure system having a tarp cover, a plurality of substantially U-shaped bows connected to and supporting the tarp cover, and rolling carriages connected to lower ends of said bows and which roll along the support rail, the support rail including a first elongate rail section which is fixed to the trailer and extends substantially parallel to a cargo support surface of the trailer, a second elongate rail section which is movably connected to the trailer, and a support mechanism which connects the second elongate rail section to the trailer such that the second elongate rail section may be moved between a first position extending continuously to the first elongate rail section along a common axis and a second position parallel to but at a different level from the first elongate rail section.