Well, I am going to follow NWL with MY "new" American Flyer set, but its from (about) 1924 so its 8 years after the beautiful set posted by NWL above.
Here is the Type XI engine with "A.F.-16" under the cab with embossed lettering in the casting and highlighted in gold. It is without the raised panel hence dating it from 1923 at the earliest. Despite being nearly 100 years old, the CW motor works perfectly and so does both the cab brake lever and the under the frame brake trip which works with a brake rail in the track (which came with the set).
Here is the entire train. IAW Greenberg's AF Prewar O'Gauge the "new" 6 inch cars were introduced in 1923 or 1924. He states that there is at least one verified set, just like this one, from 1924 so the book maintains that is the true introduction date of these new cars. The Pullman is marked "Chicago, Milwaukee, and St. Paul" over the windows and 1206 - Seattle-1206 under the windows. The 3-window , 6 panel door, mail / baggage is marked American Flyer Lines and 1205 on the left side of the door and "United States Mail / Railway Post Office" in two lines on the lower right side of the door. Both have (using Greenberg's nomenclature) Type lV trucks and Type V couplers (garter slot) . The color scheme and lettering date them from about 1924
Here are the Pullman and Mail/Baggage car in close up.
Here is the set in the "bottom" of the set box. Alas I don't have the cover and I can't really prove this is the bottom of this set box. However, this one is a robust well constructed box both glued and stapled. the dividers are the right size for the pieces and are also robust and permanently fastened into the outer box, so this clearly was a train set box, I just can't prove that it was for this set.
Although I am not as familiar with early AF CW track, these pieces appear to be genuine and of the same time period as the cars. As delivered to me, the "set" only had straight track (which of course could not be its original configuration) so I don't know if the curved sleepers are canted or flat. The condition of the track is excellent however and consistent with the condition of the trains.
Hope you enjoy the pictures, this set was a thrill for me to find and acquire.
Don