Balance flow constraints: dictate that all cars flowing into a transshipment facility should also flow out of the transshipment facility - i.e. no inventory of cars should remain at any of the transshipment facilities. Inflow=Outflow or Inflow-Outflow=0. In other words, sum of what quantities going in should be equal to sum of what quantities going out for each transshipment facility. 2. Capacity constraint: Above constraint does not account for capacity of each Transshipment facility. For example, what is going can be equal to what is going out in Louisville facility, but total cars going through (sum of inflows) should not exceed the capacity of Louisville