In the Midwest, you’d see every type of freight car.
40’ boxcars were of course the most common, hauling virtually everything. 50’ boxcars weren’t that common, and were generally reserved for automobile manufacturing service (Detriot IS in the Midwest…)
Gondolas were probably the seond most common. Gons hauled everything that boxcars did, if it didn’t need to be protected from the elements. There are a lot of steel mills in the Midwest, and they hauled finished product and scrap. And most roads outside of Appaliachia actually hauled more coal in gondolas than in hoppers. Gons would also be used to haul lumber and non-coal minerals.
Reefers were very common. 80% of the population of the USA lived east of the Mississippi, so all that California produce had to pass though the Midwest to get to the eastern seaboard. 95% of the meat industry was in the Midwest (Chicago, St Louis, Kansas City), and all those steaks and hotdogs were loaded in to meat reefers.
Also because of the meat industry, stock cars were relatively common, especially during the two big rushes (spring to pastures, fall to slaughter)
Because of the industrial base, flats would be fairly common, hauling finished machinery.
Hoppers were more common than you think. Besides hauling fuel to coal dealers, hoppers hauled a LOT of coal to electric generating stations; better than 60% of the electricity in the USA was generated in the Midwest (Ingall’s Illinois empire). And believe it or not, there’s more coal under Illinois than there is under West Virginia; several roads hauled billions of tons of coal out of Illinois, Indiana and Kentucky.
Tank cars hauled more than oil. Chemicals, food grade oils, paint, molasses.
Finally, you’d see a few twin bay covered hoppers, lugging concrete or other keep-dry minerals around. They were specialized service, so didn’t appear in large numbers, and only went to larger companies.