But for specialty trains, like IC coal hauls or ATSF refrigerated service, etc., then you are approaching 100 percent home roads. I sure to remember those long IC coal trains with the “same” two bay hoppers going by, one after the other - all Illinois Central. It got boring, but that was the way it was.
Let’s put this myth to bed.
IC could line haul complete L&N or Southern coal train to (say) a steel mill in Chicago or to a Lake Michigan port.
Those Santa Fe reefers could be produce train for Louisville,Ky hauled by the IC.
So,we know there are solid blocks of foreign road cars being line hauled to a destination on the IC.
I sure can’t argue your point, but I spent a lot of time watching IC trains coming up from southern Illinois that were IC cars. I never saw a solid train of other road cars being hauled by the IC. Of course you say they did, and you also pointed out they were solid blocks (not mixes of different roads).
The ATSF trains I watched were pretty much the same situation, pulling blocks of home road cars. Again, they could have pulled FEC cars for all I know, but I never saw them.
In any case, for the OP’s question, you would want the majority of your cars to be the home road. That of course isn’t a rivit counter’s rule, but certainly a worthy guideline.
By the way, my watch points for the IC trains was Anna Illinois, and Joliet Illinois. For the ATSF it was Joliet.