Junkman, around here(Kansas), there are lots of old boxcars sitting on farm property. Most I've seen were used for hay storage. Initially, they might have been used for grain storage as they were fairly weather tight when newer. I don't recall any that had the wheels/carriage under them--they were set up on railroad ties and such.
My guess is this all happened in the late 40's/early 50's. I would guess they were moved by placing wheels under both ends and towing to the destination, much like you would move a house today. I've not seen any newer ones moved to the farms. I suspect any newer ones now are recycled by the RR.
Best wishes,
Ron