How Do RRs Share Cars?

Can anyone give me a simplified version of how different RRs share and keep track of freight cars? For example a CSX train has tons of different RR cars in it and a siding that is on the CSX mainline has NS cars on it. How does all this work?

There are serial numbers on the sides of all the cars on the rails which can trace back to who owns what.

Each train has a manifest which lists the freight cars, their weight, any haz-mat, etc. The railroad's master computer keeps track of where each car is, and where it's supposed to go. In theory. So, a customer can call up to ask about the tank car of molasses which was supposed to be delivered yesterday, and a clerk will tell him "It's at Selkirk yard, and should be delivered to you tomorrow"

Sometimes cars are in dedicated service, so they shuttle back and forth between two points. Like the Tropicana Juice train. Sometimes a shipper will hold onto cars to assure they have cars when their goods are ready to go. Or cars are in special service hauling sulphur, so they wait to be loaded.

Coal plants will often get 100+ car trains of coal, and dump them over 5 days. At the end of the 5 days, the railroad will deliver new loads, and retrieve the empties

And they also get paid for days that the car is on another railroad, called per diem. This is a set fee charged for the use of the car. It also explains why many small railroads have large freight car fleets, as a secondary source of income. I remember reading a book on the Interstate RR where they had trains whose sole purpose was to get the cars off their railroad before 12 so that they would not have to pay for cars they did not own and begin to be paid for cars that they did.

When I worked for C&O/B&O we would notice how the Southern would expedite their northbounds into our Cincinnatti yard every night (putting their dispatched southbounds into sidings down in Kentucky) just so three or four trains of about a hundred cars each would get off their road at midnight. In the 60s the per diem cost per car (loaded or empty, by the way, it didn't matter) would range from $4.50/day (more like $12-13 today) up to about $20/day ($60 today). Multiply these figures across 400 cars or so, and that added up to a chunk of change.

At the end of the month there would be a big accounting balancing out among the roads.