Yes the RPO Terminal Clerk, while non-moving is part of the RPO system with a direct chain of command up to the level of Assistant Postmaster Generals.
City Postmasters have a chain of command to a different Assistant Postmaster General.
Thus the transit clerk, while located within the city delivery limits is not considered part of the city as the City Postmaster has zero jurisdiction. So mail deposited within Albany, NY but picked up and cancelled by the on site RPO Transit Clerk did not qualify for the local carrier city rate.
...Lastly, the United States Official Postal Guide, Part 1, Fifth Series, Vol. 3, No. 1, July, 1941, p. 128, states:
27. Posting letters at railway post offices—The privilege of
posting letters in mail cars and in letter boxes at railway stations
from which collection is made by railway postal clerks is intended
only as an accommodation to travelers and to permit the mailing of
a few late letters which cannot be deposited in post offices in time
for their intended dispatch....
This subject was covered in detail in a 2018 Article, "When 'Local' Isn't" in The Specialist published by the USSS (United States Stamp Society) with appropriate citations and included the above quotation.
I apologize for my failure to know which month is was published but I do know it was not January nor February.
Perhaps one can post the month if they know it.
Edited to include the listed quote.