Summation of answers and comments:
Use "an" before a word that begins with a vowel sound,
otherwise, use "a".
Still not clear on which to use before acronyms that can be read as letter-by-letter or as the words they represent.
Side note: In the future, I'll only use acronyms where there is no need for "a" or "an" before them. If an "a" or an "an" is needed, I'll write the words of the acronym.
I'd use: "Nightmare of an operator of N.Y.C.'s subway/Subway..."
Simpler sentence:
Instead of "An NYU student" or "A New York University student",
"A student of NYU" or "A student of New York University".
Original Question:
I'm pretty sure I use the correct "a"/"an".
Something like: if the next word (expanded from acronym, if exists) starts with "a"/"e"/"i"/"o"/"u"(/"y"?) => use "an", otherwise => use "a".
A (bad?) example from "The New Yorker"(!) on YouTube:
"An N.Y.C. Subway Operator's..."
What are the "official" rules?