It would seem like that once the forward rail head/base sent supply trains to an army that most of the wagons never made it back to the rail head/base. The armies would have broken down or damaged wagons and would have grabbed any wagons they needed. Also the mules pulling the wagons would have been needed to replace sick , injured or worn out mules. This would have left few mules to return the wagons to the supply bases.
Has anyone seen the percentage of wagons and mules/horse that returned back to a supply base?