A solarium was much more practical than the heavyweight and some lightweight cars at the time of construction. Why? It could be mounted mid-train if need be with the additon of a diaprham and walkway equipment.
Also, while many have ideas of riding on the back of an open-end observation as a train bombs down the track, he quickly reminded me of why one gets a "salty taste" on their lips after a few miles!

Somewhere I have some CP passenger car books, if you want to know more about specific CP cars, I can dig...