Model Railroader ran a good story on this a few times, most people that are into prototype operations use the following as a guide to what runs on their layouts. I modeled the Soo, so about 55% of my cars should be Soo, the real railroad wants to haul their own cars as long and as far as they can. But you mix-up that 55% said if your modeling the 50’s about 38% of your cars would be boxcars, then 15% gons, 5% flat cars, 25% refers,some coal cars. Then your going to have 35% of other midwestern cars from interchange, CB&Q, Milw RD, CNW, Rock, etc.
The rest of the cars would be a mix of east, west, and southern cars. But now model the Soo in said 1980, your going to have about 10% boxcars and 35% Covered hoppers for grain, why?
1950 almost all grain moved in 40ft boxcars by 1980 almost all grain was in covered hoppers and the 40ft boxcar was in anew car somewhere.
See it just depends on you if you want to run 20 different roadnames on one train thats up to you. Thats whats nice about this hobby your free to do what you want not what Joe Blow wants. But if you go to someones layout and you see that they are running a more Prototypical
mix of cars you can bet they have proably done alot of reseach to find what the railroad they model really ran and to where.
Also starting in the mid 80’s came the leasing boom, where some real railroads that only had said 5 miles of track, owned 500 50ft brightly colored boxcars and they where all over the place. This made it so the big railroads like BN, NS no longer had to own and repair all those cars themselfs.
Theres alot more to it then just running a train around in a oval, if you want there to be. Railroading is the most researched industry there is, there 1000’s of books out on own the real railroads operate.