I have seen this question and others like it here more than once over the years. There is sometimes a misunderstanding about how the US (and other) rail systems work.
All railroads (except some industrial roads, and the like) are interconnected. They do what is called "interchange". Behind your ICG (or any road) locomotives you could find, actually, any railroad in the US (and Mexico and Canada) that interchanges with other roads. Essentially, all roads could show up - and did.
Certainly there is a tendency to see cars from the home road and others that have more interchange traffic with it. Also, since railroad A bills railroad B time (demurrage) charges, railroads want to send a railroad's card "home" as soon as possible, home road cars tend to be the most common.
Also, there are "private line" cars - those cars who's reporting marks end in "X" (like "GATX"); these are not railroads (they may be owned by one or more railroads) and tend to be in somewhat "captive" service to a mill or refinery or the like. You could see many of these as used by shippers on your road.
Basically, so long as the era is correct - your time frame or earlier, and sometimes a lot earlier - anything could show up behind your orange and white locos - but, some are more likely than others.
Also, big railroads tend to have a lot of cars, and these show up more often all over the place than those of small railroads, unless there is a regional reason. A steam-era model RR would have lots of ATSF, PRR, NYC, UP, and so on cars, regardless of where it was set.
Interesting. I worked for the GM&O/ICG, in Mobile, for 15 years.
