A drilling rig may include a pair of main beams supportable by a pair of rails, a substructure, an A-frame secured to the main beams, and a mast pivotably secured to the main beams and configured to lay down in a pre-erected stage. The substructure may include a plurality of pivoting legs, a drill floor having a plurality of spreaders pivotably supported by the plurality of pivoting legs, and a plurality of drill floor subassemblies supported by the plurality of spreaders. The plurality of pivoting legs supporting the spreaders may be configured to lay down in a pre-erected stage as well.