The tractor in this story is a murder weapon. The neglect and silence of Mrs. McIntyre, Mr. Shortley, and Sulk allow the huge piece of machinery to roll over Mr. Guizac's leg and snap his spine. The tractor is also a symbol of the money Mrs. McIntyre is making as a result of Mr. Guizac's labor and the money she probably already had, even though she constantly complains about being poor. Tractors and other farm equipment are expensive and Mrs. McIntyre keeps investing in more machinery throughout Mr. Guizac's time on the farm. Since Mr. Guizac is skilled at operating, repairing and maintaining machinery, the tractor also represents his talent, the talent that he loses along with his life.
Generally speaking, the whole tractor issue might also express anxiety over increased industrialization, from the various perspectives of farmers and farm employees in the rural South.