Milk trains consisted of tank cars and can cars..... sometimes one or the other and at others both kinds. Tankers were came in two forms, a tank which sort of sat on a flat car base, and the enclosed tank car such as the one you ordered. Milk tank cars usually were loaded from tank trucks which collected the milk from various dairy farms.
As to railroad operations with these tank cars ... I'm not quite sure if these cars were loaded from a team track or spotted on a siding so they could be loaded by several tank trucks, and later being picked up by the local milk train and hauled to the processing plant.
