Sorry, I had intended to make more of my previous reply but I had a train to catch. Anyway, as far as the preponderance of co-op cars in grain trains is concerned, I was really speaking to BN unit grain trains of the late 80s/early 90s. If you're looking to sprinkle regular covered hoppers into your mixed freights then the co-op cars could work, but like you suggest "regular" cars might be a better fit. This could really turn into a rabbit hole, but if you have an idea of the on-line industries your model railroad serves as well as the origin and destination industries that correspond to those industries it can make choosing appropriate cars a little easier. Likewise, if you have an idea of the traffic your prototype handled across the modeled portion of your railroad you can choose appropriate cars even when they won't visit an industry on your railroad. Here's an example: I am not modeling the General Motors Arlington Assembly plant, but it is located very near the prototype I'm modeling. I know that during my era the plant produced G platform coupes, so trilevel autoracks were plentiful in trains headed to and from the plant. GM favored the four door per side jumbo boxcars, so those also need to be represented in the trains along with appropriate 50' and 60' boxcars. Another example is a brewery that is not going to be part of the modeled railroad but whose traffic is important locally. In my era they received corn syrup (corn before that) and malt and shipped product out in insulated boxcars. The packaging plant received paper/containerboard and the can plant received aluminum canstock. So cars representing this traffic are part of the UP mixed freight consists. Unlike modeling a unit grain train a few appropriate cars here and there get the point across and it gives a purpose for those CNW hoppers or UP family beer cars.