Some ideas on milk...
Firstly am I correct in presuming that by the term "churn" you mean the classic (around here) metal cans; about 16 inches (or so) in diameter and 30 to 36 inches tall, two handles at he top and not painted?
Secondly, always remember there are two parameters to any load, volume and weight. If I remember correctly Trainz uses volume units multiplied by a weight per volume figure. For example; the car can hold X gallons (volume) and milk is about Y pounds per gallon, resulting in a product weight, in pounds, of X*Y. It is entirely possible to load out a car's weight rating and not its volume as well as the reverse volume and not weight. An often overlooked point, milk "churns" (or cans) are round, so there is some lost space when placing them in a square container. The quick and dirty way around this is to assume that you place them like hexagons, and have a missing part can at the end of each row.
Thirdly, in New England milk was a big commodity in the first 60-70 years of the last century. The Maine State Museum had a good exhibit (rotated out unfortunately) on the whole process, not much on the rail transport though. Most rail transportation was from small local creamery facilities to the larger facilities in the cities (Portland, Boston, etc.). As the road network improved in the area the use of rail transport understandably declined. However the area railroads ran many milk trains until about the late 40's, using specialized head end equipment on passenger trains. The most common equipment around here for cans was the insulated express boxcar not the express reefer, however many baggage cars were also insulated. In New England there was also the problem of keeping the milk warm enough in the winter. "Insulated" cars seem to have been preferred over "refrigerated" cars however this may be due to semantics or designation by the railroad. Most of the boxcar looking glass lined milk tankers were only insulated. A note to the "yeah right" crowd, a partial car of milk will keep itself cold (or warm) enough as it is a mater of insulation and the cold milk provides the cooling effect.