Arnold,
In addition to all the great answers from the "real pros" here on the form, let me tell you about one of my favorite pastimes. I have a small library of books with marvelous pictures of Northeastern railroads (many by Morning Sun Books) and I always scrutinize every car in these photos, especially yard shots and freight service. Sometimes I have even taken to using a magnifying lens to try and make out more distant, out of focus elements of pictures! I am always amazed at the variety of roads showing up 'far from home' and the various types of rolling stock. Growing up, this also helped me to learn a great deal about US geography (e.g., "Route of the Zephyrs? Soo Line? Where is that? What is that!"). Such "sloganeering" or just plainspoken origins ("Toledo, Saginaw and Muskegon") were, and continue to be, gateways to my imagination and interest.
Some folks have presented various basic formulas such as 70% home road, 30% other for mixing up freight consists, which seems to work well. Nowadays, I don't worry about it so much--if I like a car (or a "unit"), I'll run it. That's playing by my homefield advantage!
