"Kentucky Railway Museum has the 10 section Solarium Observation Car Mt. Broderick
Midwest Railway Preservation Society in Cleveland has Mt. Baxter of the same design.
Old Dominion Chapter NRHS has Pullman Dinwiddie County
Rail Giants Museum has NKP business car number 6, the former Carleton Club
Sister car Kitchi Gami Club is privately owned.
Heber Valley has a solarium car of Canadian extraction. I'm not sure if it's of CP or CNR heritage."

Not to call out Mr. Wilkins in particular, but most if not all of the cars he mentions are simply enclosed (as opposed to open) observation-lounge cars, with various sleeping accommodations. A solarium or sunroom car is characterized by the high windows (with transom sash) mentioned in Howard's post, and shown in Timothy's photos. Other typical "spotting features" of a solarium are the absence of a rear-end diaphragm (at least as-built), a short brass-railed gate across the end opening, and a pair of narrow double end doors that could be opened to create something of the feel of an open platform for travelers in the short solarium area, which was typically furnished with wicker chairs and separated by a bulkhead from the main lounge. "Sun-parlors", sunrooms, or solariums, similarly furnished, were trendy features of nicer homes of the 1920s, and Pullman appears to have simply been following that fashion.

There were also open-platform observation cars with high (non-transom) windows throughout the observation lounge, such as the NYC-assigned "Valley" series cars used on the 20th Century Limited. Some were later rebuilt with vestibules having deep windows in the end sheets and special deep-windowed vestibule doors, while retaining their former observation platform windows, all amounting to a see-through vestibule and some of the appearance a solarium car, but without any actual solarium seating area.

This car was built by the Pullman Company, lot 4101, as their car “Holton”. It was built as a 16-section sleeping car, plan 2412A, and was released for service in February 1913. The “Holton” was part of a 20 car series and was built for train “San Francisco Overland Limited”. In May 1931 the Pullman Company rebuilt the car into a buffet lounge car with sunroom; plan 4006B, for the Western Pacific Railroad as their number 653. The WP leased the car from Pullman for five years. In September 1936 the Western Pacific purchased the car from The Pullman Company for $38,629.97. It was used on the train “Exposition Flyer”. The car was part of a three car series, 651-653.

The car was built by Pullman Standard for the Pullman Company, lot 4528, as their car “Brady”. The car was part of a thirty car series “Balfour to Buttrick”. It was built as a 16-section sleeping car, plan 2412F, and was released for service on December 19, 1917. It was built with type LN air brakes and one 18-inch brake cylinder. The Pullman Company rebuilt the car into an eight section, buffet, observation car, plan 4025. It was released to service in this configuration on October 28, 1932 as car “Circumnagavigators Club”. The car was intended for service between St. Louis and Minneapolis-St. Paul on trains operated by jointly by the Wabash and Minneapolis and St. Louis Railroads. The rebuild included changing the air brakes to type UC. Ice air conditioning was added to the car on April 26, 1934. The interior plan was changed to 4025N on July 8, 1943. The significance of this change is not known. The color of the exterior of the car was changed from Pullman color (dark green) to two tone gray on August 30, 1954. The Pullman Company retired the car from service and sold it on October 1, 1965.