I've recently acquired a set of N Scale 6-axle Heavyweight cars in C&NW Green/Yellow. They are apparently a 30-year-old set produced by Con-Cor, and I'm not sure if they represent a true prototype or if they're a complete fabrication. Would any of these cars be a reasonable stand-in for something you might have seen in Chicago-area commuter service prior to the arrival of the bi-levels?
Quite nice, and they seem like reasonable facsimiles to me but then, I wasn't around to see these in actual revenue service (despite what some believe); so I am no means an authority. A more reliable source, the Chicago and North Western Historical society will be much more helpful in providing answers to your question.
Happy modeling!