Brad certain roads preferred 36’ cars up to the end of livestock movements. Attached is a spreadsheet I created some time back that shows stockcars for the CNW, CGW, Omaha, & M&StL showing the years 1941, 1943, 1953 and 1960. It includes their length. Noticed the dominance of 36’ cars for the CNW and Omaha roads. I think you will see similar preference for 36’ cars on the CB&Q and MILW. As suggested an ORER will quickly show the information you seek. I’m not at home, so not able to access the copies I have.

Attached are a few photos.