Wow. Learned lots this evening from reading the awesome Dispatch issue that Big Jim shared. It answers just about every imaginable question about operations, and MWB's link has excellent information and photos about the area I currently live in (although I'm not modeling the "milk era").
Dominic, based on what I just read, private-owned dairy cars had the twin tanks, while RR-owned milk cars shipped the 40-quart cans. It's incredible that ice wasn't needed for the tanks because the sheer volume (and insulation) kept the cargo cool. No mention is made of AmEx or REA per se in the article, but front-end baggage cars were used. So, maybe? As reefers, it seems it would be hard to say whether these would contain cans or the tanks. I'd guess the former because of their use for shipping fruit and etc.
If this difference in tank vs. can shipping method holds true across all private/lessee vs. RR owners, then my on order B&O car should not have the twin tanks. It's a good thing they are removable :-).
Back to the baggage cars: my current town was a milk producer for the B&M. One surviving photo shows dairy farmers in front of an open car door with the cans stacked two high inside, as is typical. But what's interesting is that there are two glass paned windows on the side of the car. Glass windows seem like a bad idea for a reefer. Unfortunately, it's a close-up photo and that's all that can be seen. I have a B&M-specific photo book that might provide more clues.
MNCW, that's an interesting sounding book you've mentioned, and Rule 292, thanks for the pointer to the 2-rail post; I'll definitely follow-up on that soon. (It's getting late.) Also, Lionel is making a Supplee car along with the B&O. Your post is the first I've heard that name.
Mike CT, you've provided the "micro-micro" milk operations details! 
Dare I ask how you came to know all of that? When I lived on a farm, the farm mom would hand-milk Beulah first thing in the morning, pour the milk from the pail through a paper filter to remove bits of the cow's behind (if you catch my drift), then pop the milk in to the institutional-sized milk dispenser in the kitchen for us all to drink. No bacteria testing, no temperature monitoring, no weighing, and I've lived to tell the tale! 
We couldn't drink it fast enough, so farm mom decided to make cheese -- in the house. Yuck! curds and wheys stink. But that's an operations story for another time.
Cohighrailer, good luck with the truck tanker kit bashing project. It's great you found a model in the time frame of the pickup truck you rode in. I think I will just pretend I have a tanker with a nice Divco milk truck. Did you see in the Dispatch article that interurbans were used? (There's no mention as to whether they were modified.) But, it seems possible that they could have seen use for milk in Denver. Is there an "Images of America" photo book for Denver? That book series has proved useful for answering many questions.
Tomlinson Got Milk? RR
