Abstract: A trailer for selectively transporting either a first or second substance is disclosed. The trailer includes a substantially flat bed and has two containers disposed thereon, each of which is capable of storing a different type of substance. An operator may choose to transport materials in either one of the two containers. Accordingly, the container to transport the substance is shifted toward a balanced load distribution point along the trailer while the empty container is moved to one end of the trailer. In the preferred embodiment, the two containers are coupled together so that the movement of one container along the trailer automatically causes a corresponding movement in the other. One container moves along the trailer by virtue of a plurality of wheels while the other slides along rails separated by a bearing.