Matching waybills to cars is done anyway you like. The restrictions are that the car should match the load -- no coal in tank cars or grain in the stock cars. Pick a set of waybills and match them to car cards. Then divide them up by trains depending on where they're going. You can get more sophisticated by making the cars that certain waybills match with more restrictive. It may help if some waybills start or end up in a holding track somewhere. You may not have both industries for a waybill e.g. no steel plant to supply raw materials for your washing machine plant.