This is such an interesting and informative thread! Thank you to all who are contributing information.
I grew up in Southern California, where dairies were concentrated in Artesia, Norwalk, and Cypress, about 15 miles from downtown Los Angeles. This territory was served by Pacific Electric Railway. The area produced a very great amount of milk, but I have never heard of any milk trains on the PE. I believe that the distance to the processing plants was so short that there was no need to ship carloads of milk. Also, California was very early in building a good network of paved roads and highways, so it was possible (and faster) to use tank trucks instead of milk trains. However, old head Conductors I worked with on Santa Fe told me that, when they were working Baggagemen assignments, they routinely loaded dozens of milk cans in Santa Fe baggage/express cars, mainly from small dairies along the Second District (LA-San Bernardino via Pasadena/Azusa/Pomona). And I would feel safe in presuming that PE hauled a lot of milk cans in its express box motors. But the large dairies used trucks. California was populated a hundred years after the northeast, and therefore had the advantage of building many dairies in southeast Los Angeles County and southwest Orange County, instead of scattering them throughout the region.
Around 1960, the progress of suburbia made the dairy land too valuable for agricultural use and the farms moved farther out. But, by that time, large trucks and stainless steel trailers could quickly transport the milk, and there was no more rail transport of milk.
In the northeast, upstate New York and Vermont were settled much earlier and dairying was well-established on family farms that were scattered more widely than the dairies where I grew up. Thus, collection and shipment was done differently.
