Era?
Probably the answer prior to the 1950-1960's was a boxcar.
I know that in 1956 on my line Bethlehem Steel opened a taconite mine on my line and recieved shipments of bentonite clay in 2 bay covered hoppers. Since fuller's earth is similar, after the 1950-1960's that would be an option also.
Looking at the plant today, it appears to ship in covered hoppers.
https://www.google.com/maps/@30.601996,-84.572335,3a,75y,253.4h,90.62t/data=!3m6!1e1!3m4!1sq6ihCn0LM6TMkfK_BnZf1Q!2e0!7i13312!8i6656
The picture of the clay being shipped by rail was probably from the mine or pit to wherever it was processed.
Dave H. Painted side goes up. My website : wnbranch.com
Not sure on a time period your talking, but these cars look like a shop made kind of ore car.
https://www.floridamemory.com/items/show/28644
Mike.
My You Tube
Does anyone know what type of cars are used for transporting fuller's earth? Specifically, I'm talking about the Floridin Company in Quincy, Florida.