The Great Western Railway, like all of the major railway companies on the pre-nationalised British railway system, had a large fleet of freight wagons carrying a huge variety of loads. The bulk of the fleet (which numbered in the tens of thousands) carried standard loads like coal and other minerals, milk, food stuffs and finished goods.
The GWR also had a number of specialised wagons for special loads which could often be hazardous. These might have better suspension and shock-absorbers, or cooling/heating systems. Gunpowder vans traditionally had metal bodies with a wooden interior (to reduce the chance of sparks of course, potentially catastrophic on a train load of gunpowder!) Care was especially taken with the doors and no steel on steel contact was allowed. Brass was used for all hinges and fasteners and the wood interior was also lined with lead.
GWR 58725 is a surviving example of a gunpowder van which currently resides on the Severn Valley Railway. It was originally built as a standard iron Mink van at the end of the 19th century but later converted into a gunpowder van.