I would assume hops would be carried in hopper cars or in boxes in box cars. I'm not familiar with British railroading, but I can give you my guess.
In American railroading, hopper cars are cars that carry many items. Hopper cars are generally placed in two classes; covered hoppers and non-covered hoppers. The covered hoppers ship items that don't need to be covered like coal, ballast (rocks in between the rails), and plastic pellets. Covered hoppers carry items that need to be covered like grain.