Overnight express freight trains from the milk producing areas of Britain to the big cities. GWR milk trains arrived each morning at around 5am into London from Devon and Cornwall (for example) permitting fresh milk to be on the breakfast table or in the store at Sainsbury's. According to the Sainsbury's website they took pride in having the freshest milk on sale in London. Milk travelled better overnight as temperatures were cooler. It was also important that the train maintained a constant speed or else by the time the milk arrived at its destination it would have been churned into butter!
Milk trains would return home empty during the day but of course it would then be a stop go slow freight job and there could well be a few revenue earning parcel vans and freight vans added to the consist forming a mix freight train.
I suppose that because they mainly travelled overnight milk trains (like postal trains) were rarely seen by the public.
The refridgerated milk wagons were introduced to permit daytime running of milk trains.
Happy modelling
Gary