Does anyone know the railroad that served the Pullman Car known as the SUPERb in 1920?

This, of course, is the car in question, preserved by the Southeastern Ry. Museum in Geirgia: http://www.srmduluth.org/Features/superb.htm

As to the specifics of where it was running in 1920: darned if I know or can find out.

I looked up the car SUPURB on the database located at Tom Madden's "The Pullman Project" webpage. Of course, this contains the mechanical, naming and paint history of the car and not the service history.

The car went into service on March 6, 1911, and was sold to the Charleston & Western Carolina on June 5, 1945. Between 11/4/25 and 7/7/26 it carried the name LOS ANGELES. The PM brakes were replaced with UC brakes on 6/3/26. The side sheathing was rebuilt (removing the upper Gothic style windows) Dec 26, 1928.

Given that the car was used for two presidential specials, I think it is likely that the car was not assigned to any one railroad for much, if not all, of its life. It may have been configured as a private car from the beginning and available for short or long-tern lease from Pullman, not regularly in the consist of any train?

Yes. How that I'm home I can look up some more information.

SUPERB was built to Plan 2503 as was a sister car IDEAL. These were private style cars that Pullman leased out to individuals (or corporations) that wanted to travel in "style" without owning their own private cars. Pullman had a whole fleet of these cars. Early ones were wooden, then in 1911-17 steel ones were built and then more steel cars in 1927-8.

These cars were removed from lease at the start of World War II. Most were then sold to individual railroads.

Robert Wagner's The Complete Roster of Heavyweight Pullman Cars says that in 11/25 the car was renamed LOS ANGELES, 6/26 the car was renamed POPE PIUS XI and then in 7/26 back to SUPERB. Sold 6/45 to Charleston & Western Carolina RR becoming business car #101.

Now going to Arthur D. Dubin's Some Classic Trains on pages 380-383 there is a section on "The Cardinals Train." This was a special train that operated in June 1926 from New York to Chicago and return for the XXVIII International Eucharistic Congress held in Chicago June 20-24. This train was made up of Pullman cars only with private rooms (no section space) and the cars were painted red. It carried Cardinals of the Catholic church to this meeting. The private car named POPE PIUS XI carried Cardinal Bonzano who presided over the Congress.

And, oh, only now have I looked at the linked page.