(countable, uncountable) The subway lines, collectively, formerly run by the Interborough Rapid Transit Company.
A Soho sweetheart wants the man who injured her husband thrown in front of the IRT. 1977 April 30, Steve Blevins, “A Sack-full of Films”, in Gay Community News, page 11:
Take the IRT to Atlantic and change for the R.
(abbreviation, alt-of, countable, initialism, uncountable) Initialism of item response theory.