Up until the mid-nineteenth century much of London’s milk was supplied  by cows kept in cramped urban locations. This proved unhygienic and as a result of a cattle plague this practise diminished after 1865. Around  about this time the milk started to be transported into London by rail  on frequent express trains.  By the 1930s the volume of milk being transported had grown to a high  enough volume that it was no longer deemed efficient to transport using  traditional goods vehicles. A as a result United Dairies ordered a  series of tank wagons, with six-wheeled tankers proving to be the most  effective design although four-wheeled wagons were also used. United  Dairies ended up operating over three hundred such tank wagons.