I've tried searching for an answer to this, but apparently I just don't have the right search terms- I've found all kinds of answers as to exactly what rolling stock looked like in terms of actual cars, even the ways car types have changed over time (such as the removal of full ladders on boxcars), or what percentage of which car types are in a given railroad's roster.
However, what I'm looking for is more of "car types and proportions" in the average cross-country freight run. I live on the Colorado joint line, so there's all kinds of traffic passing through from every direction, and I've pretty well got a handle on what the freight looks like since around 1995.
But what, for example, would a through-freight look like on an average day in 1955, 1965, 1975, or 1985? The first layout I'm planning is focusing on steel mill industry based on the Pueblo, CO mill, but I'd like to have some trains that "pass through" on occasion. I'd like to set the "year" of the layout back when the mill was in heavier operation (definitely when the blast furnaces were running), but I haven't been able to find any general guidelines on what an average train might look like.
Any assistance would be greatly appreciated.