A modular data center system is disclosed for constructing a modular data center building. A plurality of structural support columns are used to support a plurality of unit structures above a floor surface at a desired height relative to the floor surface. A pair of elongated support rails are also included which are coupled perpendicularly to the plurality of structural support columns, at approximately the desired height, to form two parallel, horizontally spaced apart tracks. Each of the unit structures includes a plurality of carriage assemblies, with each of the carriage assemblies including a wheel. The wheels allow the unit structures to be rolled horizontally into a desired position on the pair of elongated support rails when constructing the modular data center building to expedite its assembly.