The Interborough Rapid Transit Company (IRT) was the 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. The IRT was purchased by the City in June 1940. The former IRT lines (the numbered lines in the current subway system) are now the A Division or IRT Division of the Subway.
The first IRT subway ran between City Hall and 145th Street at Broadway, opening on October 27, 1904. It opened following more than twenty years of public debate on the merits of subways versus the existing elevated rail system and on various proposed routes.
On April 1, 1903, over a year before its first subway line opened, IRT acquired the pre-existing elevated Manhattan Railway by lease, gaining a monopoly on rapid transit in Manhattan. The Manhattan EL was the operator of four elevated railways in Manhattan with an extension into the Bronx. The IRT coordinated some services between what became its subway and elevated divisions, but all the lines of the former Manhattan EL have since been dismantled.
Today, the IRT lines are operated as the A Division of the subway. The remaining lines are underground in Manhattan, except for a short stretch across Harlem at 125th Street and in northernmost Manhattan. Its many lines in the Bronx are predominantly elevated, with some subway, and some railroad-style right-of-way acquired from the defunct New York, Westchester and Boston Railway, which now comprises the IRT Dyre Avenue Line. Its Brooklyn lines are in subway with a single elevated extension that reaches up to New Lots Avenue, and the other reaching Flatbush Avenue via the underground Nostrand Avenue Line. The Flushing Line, its sole line in Queens, is entirely elevated except for a short portion approaching its East River tunnel and its terminal at Flushing–Main Street (the whole Manhattan portion of the line is in subway). The Flushing Line has had no track connection to the rest of the IRT since 1942, when service on the Second Avenue El was discontinued; today, its sole connection to the rest of the system is to the BMT at Queensborough Plaza.