I think all the remaining sets have an observation car. Most people opted for a set with the observation car because they are quite beautiful - though not prototypical. (Very few sets were done with a correct 14-4 in the set.) Many then added the 14-4 as an “extra”.
There are only 2 #ed observation cars. It’s nice to get both #s.
The observation cars ran only on the Missouri Pacific Missouri River Eagle between St. Louis, Kansas City, and Omaha. The inclusion of the cars in the sets was a concession to demand for the cars.