The Interborough Rapid Transit Company ( IRT ) was the first operator of the original underground New York City Subway line that opened in 1904 , as well as other earlier railways and rapid transit lines in New York City . The Interborough Rapid Transit Company ( IRT ) was the private operator of the original underground New York City Subway line that opened in 1904 , as well as earlier elevated railways and additional rapid transit lines in New York City .