'Tail Traffic' is the name railwaymen use to describe non-passenger vehicles (eg. mail and parcels vans) attached to passenger trains. Moving goods by scheduled passenger trains was much faster and more predictable than waiting for slow goods trains, so was ideal for highly perishable goods like milk and premium rates wre charged for this service. The six-wheel milk tank was specifically designed to travel safely at express train speeds, so could be moved from country dairies by a local passenger train, then coupled to a mainline express, swiftly delivering milk to major cities.
Modelling milk traffic adds action to your model railway. Even if you don't model a dairy on your layout you can exchange tank wagons between branch trains and long distance expresses.