The railways were an ideal means to supply fresh milk to Britain's growing cities in churns, direct from country farms. The milk needed to be kept cool throughout the journey and the initial solution was to provide slat-sided vans to create a flow of air around the churns driven by the movement of the train. As train speeds increased these 6-wheeled vans were built for improved running and stability. Later vans featured louvre vents, making them better suited to service as general parcels vans as well as for milk, with the milk traffic itself being moved to bulk tank wagons in the 1930s.
The new O5 diagram were similar to through slightly taller than the previous design, thought to be to allow more headroom for lifting churns onto a second row but lot numbers suggest these vans may have been intended for fish traffic. The same design of van was also built with end as well as side doors, this allowed road vehicles to be loaded and eased the loading of large sections of scenery for travelling theatrical troupes, adding to the flexibility in loads for the Siphon van.
As the GWR index of diagrams was drawn up the milk vans were allocated a series with the prefix letter O and the telegraphic code word for a milk van was Siphon. A suffix letter was used to describe the general type and load capacity of the van, eg 4 wheels, 6 wheels, bogies, end doors, through gangways, etc. The code word and letter were frequently painted on the outside of the van. Standard livery throughout most of the GWR era. was chocolate brown with straw yellow lettering.
The development of the milk van into the GWRs standard general utility van with the 50ft twin bogie and gangwayed 'Siphon G' design led to GWR non-passenger coaching stock being referred to as 'brown vehicles' and the term Siphon being so commonly used.