I don't think that any US roads were ever mandated to service lightweight branchlines into the 1980's, and any which did, probably did so because there was additional traffic apart from grain, although not enough to facilitate a rebuilding.
Here in Canada however, the Canadian Pacific signed an agreement in the early part of last century (someone might know for sure and correct me if i'm wrong) with the government and grain farmers, setting a fixed rate to move grain to tidewater. This was called the Crowsnest Pass Agreement and it called for the railway to haul the grain at this fixed rate (The Crow Rate) for perpetuaty, in other words, forever.
Its a sure bet that when the rate was set, the railway was taking the grain farmers to the cleaners, however, as the years passed, operating costs for the railway increased and, eventually it became unprofitable, more so, on lightweight branchlines where grain was the only traffic, labor costs to ship by boxcar were excessive and the cost of rebuilding these branchlines was even more excessive.
The railway went to court to have the agreement repealed but it took several years, during which time they were required to continue servicing these branchlines.
Once the agreement was repealed in the early 1980's, CP arranged to have grain delivered by truck to reloads on the line where the heavier grain hoppers could load it and CP then started to abandon these lightweight branchline and the 40ft grain boxcar fleet passed into oblivion.
I'm not sure if the CN was part of the deal and, again, maybe someone who knows more about the subject can clarify this and maybe even correct me if I have made any errors or omissions.
The grain was loaded from the elevators by means of a pipe that could be "aimed" in order to direct the grain through the top of the grain door into the ends of the cars first and moving toward the center.
Cheers
Terry