Was it actually the Princess Christian hospital train, or were they two different trains?
"On the 5th [June 1900], whilst helping some of the sick from the brigade hospital to the station, I had a good opportunity of inspecting the splendid Princess Royal hospital train, which is a perfect marvel of comfort for both patients and attendants. The "bunks" for patients unable to walk are detachable, and the patients are easily transferred from the stretcher to the bunk without any unnecessary or disagreeable shocks. The patient is placed near the train on a stretcher and the available bunk is detached from its sliding supports and placed alongside the stretcher, and the patient is then easily transferred from one to the other. The bunk is then carried into the train and fixed to its supports without any rude shocks, on account of the sliding system. Each bunk is fitted up with all kinds of comforts, and the train, which is of the corridor pattern, is in charge of nurses, doctors, &c. The sight is relieved by glasses of new cut flowers here and there. A cook house is in the rear of the train."
I think "Princess Royal" was a synonym for "Princess Christian" as far as Hospital trains were concerned. According to Appendix 7 of Maurice (Official History), there was a Hospital Train service. . For example, Will Bennett in "Absent Minded Beggars" tells us that the BRCS in South Africa funded one (apart from the "Princess Christian") and it was simply titled "Hospital Train No.4". Which implies there must have been several more at least.
FWIW, the Princess Royal in 1900 was Princess Christian's eldest sister Victoria, Dowager Empress of Germany. She died in August 1901, and the title then remained in abeyance until King Edward VII conferred it on his elder daughter, Princess Louise, in 1905.