Bill,
I'm certainly not a Hafner expert, but i'll contribute what i can.
To my mind at least, there are four different groupings of Hafner freights.
In the early 1920's, Hafner sold freight cars made by Bing. Bing also sold these cars at the same time, and I don't know of any way to tell whether they were sold by Hafner or Bing, so I tend to just think of these as Bing cars, not Hafner.
Starting in 1928, Hafner started selling its own line of freights, and these first cars are my favorites. They are 5-1/2" lithographed 4-wheel cars, including the "Peerless" stock car and the Santa Fe box car:
There was also a plain, black-painted flat/log/lumber car that goes with these, it uses the exact matching black frame with some side stakes riveted on.
The second freights were a matching set in the late 1930's, and they all had cream sides with red roofs and trim, and were all labeled "Overland Freight" and/or "Overland Flyer". I think these are fairly uncommon to see.
Later, from the late 1930's through the early 1950's, Hafner made what I think of as two other kinds of freight cars. One was the rounded-corner type, for which they used the same stampings as their streamlined passenger cars, so I think of these as their "streamlined freight".
The final type is the one that I most often see for sale: longer, 6" to 6-1/2" cars like the "Grand Canyon" box car, the 1010 and Phillips 66 tank cars, and the 91746 and 91876 gondolas in several color variations, as well as matching livestock, hopper, and several versions of caboose. The nomenclature got a little strange with these, as Hafner called the gondolas "sand cars", and the coal hoppers they called "gondolas", don't know what they were thinking. Collectors usually call them by their more common names, but once in a while you get a Hafner die-hard selling a hopper and calling it a gondola, something to be alert to.
My favorites are the ones I have pictured here, and you don't see them as often. They have Joy-Line-type hook couplers and the lithography and styling are very typically pre-war tinplate, and toy-like.
don't know if this helps any,
anybody with more complete knowledge of Hafner, please correct and amplify.
david