My guess is found in many of the posts above.
I do think that the steel was ear marked for the war industry. Also grain handling after wwii was by belt conveyors and screw conveyors and screw lifts. The more modern pneumatic conveying systems and storge facilities at the end users were not plentyful because lots of cheap labor was avaiable.
In the early 60's labor as a part of mfg expenses was growing and so mfg's were looking to cut cost with investment in tech items that had an ROI(return on investment). Also grain unloading/loading was not a skilled job and the labor force was starting to look toward better paying jobs.
The manual labor jobs were not very desirable. All this coupled with the problems of cross contamination of various items hauled in the box cars helped push investments in new infustructure of hopper/cylindrical hoppers, silos, sock type dust collectors, pneumatic conveying components such as the use of Gas blowers for pressure systems and various types of metering feeder devices.
Sugar is another item that has seen an impact from bags in railcars to airslide cars to liquid sugar in tank cars or grainular sugar being liqufied on rail sidings and then distributed by truck rather than shipping all that water in the rail tankers.
Some mfg's spemt their own Capital on rolling stock to move the transition to new methods forward. The mfg's also bought their own fleet of boxcars to ship their own finished goods in. But as the trucking industry was deregulated many of these mfg's shifted from 90%+ rail to 90%+ truck.
Andrew you picked an intresting subject that has had a huge impact on the rail system during my life time. Thanks for your post.
