Previously, I somewhat snidely mentioned that there were no Pullman Facts booklets about how the company managed to become a monopoly. But in fact the answer is hidden in Pullman Facts No. 10, which shows that Pullman provided a large pool of cars that its customer railroads could draw upon to deal with seasonal variations and special events.
This reduced the costs to the railroads. Since Pullman could send cars north in the summer that were serving the south in the winter, it could account for such variations with a minimum of surplus cars.
I don’t have a copy of Pullman Facts No. 11, but as the cover above shows it claimed that American rail fares were far lower than those in Europe.