Tony Thompson can answer better than I can. But if I understood his clinic correctly, the Agent wrote the empty car order and waybill BEFORE the car was spotted for loading. So the waybill, with the car # typed on it, was already completed.
Therefore, I conclude that if the industry randomly loads the car, the paperwork would have to be changed.
On the other hand, concepts like "milling in transit" or other "diversions" can certainly have no a' priori knowledge of where the car will end up.
The shipper called the agent and ordered the cars, the agent put in the car orders to the car applicators. As the cars arrived with the "car order number" on the empty billing the cars were spotted per the car order. This was important as the car order number drove the demurrage clock to the shipper. The cars were loaded, released and the shipper filled out the bill of lading/waybill. The shipper took the completed paperwork to the agent's office; the agent signed the waybill and gave a copy to the shipper. The waybill generated an invoice. The agent had the duty of checking the protocols for the routing (generally marked on the bill as "Shippers Route") and if it was all correct he had a clerk telegraph of the car movement instructions and waybill number to the billing office. The waybill copies were sent to the billing office for processing. In the days of telegraph could you imagine if a diversion was placed on the car and the car was moving faster than expected? A copy of the waybill was sent to car accounting to apply the car hire to the books and a car online was recorded as the railroads accounting department even kept records of car hire for home road cars on line as well.