Gerald-
I don’t think the notion that the railroads used revenue tank cars downgraded for MOW service is exactly correct, at least for the CB&Q.
Very few, if any, railroad-owned tank cars were used in revenue service AFAIK. I think the CB&Q cars were purchased new for company fuel, lube oil, water, and auxiliary tender purposes.
There may have been special circumstances (like the CGW’s color-coded cars) where the railroad provided cars for certain online shippers, but most general service tank cars were privately owned, leased from one of the major tank car lines (UTLX, SHPX, etc.) or one of the many small leasing outfits (GRYX- Grace Tank Car Line, etc.).
Rupert can jump in here with his insights, but it appears that from his roster work all CB&Q tank cars were either built by the railroad or bought new. There is a note that during WWII some cars were “temporarily assigned to commercial service” so my guess is that none were normally assigned to revenue use.
Charlie Vlk