This method of dry-docking was used by Willis & Hodges Gallop at their Martin's Point property to bring their schooners up for repair on land.  The rails were extremely large beams of wood...think of it as a railroad track for trains to ride on...and were used for the schooner to ride on and be brought up out of the water and onto land for repair.  At high tide the vessel was drawn up, and there left high and dry.  When the tide receded, a temporary obstruction was placed at the entrance of the "dry dock", to prevent the water at high tide from interfering with the workmen when executing repairs.  The cross members kept the sides separated [from pulling together] as the schooner was being brought out of the water.  The winds have to be just right at Martin's Point for the tide to recede so the rails are visible.