The Interboro Rapid Transit company, or 'the IRT,' was one of the two privately owned transit companies whose subway and elevated operations were consolidated, along with the city owned Independent Subway into the current New York City subway system.

The IRT got its start building the many elevated railways that traversed Manhattan in the late 19th and early 20th century. There elevated network was quite extensive, with lines on Second avenue, Third Avenue, Sixth Avenue, Ninth Avenue, 42nd Street, 34th Street, and other places. The Ninth Avenue Elevated, opened in 1869, was the first commuter transit line in America.

The IRT was awarded the contract of building and operating the first part of the New York City subway. The initial segment was completed in 1904 and consisted of the line from city hall and up Lafayette and Park avenues (part of today's 4, 5, and 6 Trains), across 42nd street, (today's Times Square Shuttle) and up the Upper West Side under Broadway (part of today's 1, 2, and 3 Trains]. Most of this initial New York City subway line was built with simple cut and cover tunneling, but it was a feat of strength for the immigrant workers who did most of the work by hand.

The IRT stations featured detailed mosaic work and nice touches like marble, and were generally located right below street level. Initially the stations were only four cars long. These were extended to ten cars long in the 50s, and in some stations the differences are apparent.

The IRT shortly expanded its subway lines, completing the Seventh Avenue Line on the West Side and the Lexington Avenue line on the Upper East Side in 1918, bringing about the familiar 'H' pattern of numbered trains in Manhattan today. The IRT also expanded its initial line through the financial district and through a tunnel under the East River to Brooklyn, and expanded under the Harlem river to elevated lines in the Bronx.

The other major transit holder at the turn of the century in New York was the Brooklyn Rapid Transit, the BRT, soon to be the Brooklyn Manhattan Transit or BMT. The BRT operated elevated lines in Brooklyn that entered Manhattan via the Brooklyn and Williamsburg bridges. The BRT built its own subway between Manhattan and Brooklyn bringing the two systems into competition.

The next period of subway expansion, in the late teens and early 20s, was the Dual Contracts era, in which the city contracted out to the BRT and IRT the work of expanding the system in routes which it determined. The IRT expanded its lines in Brooklyn to their current ends. The two companies jointly built and operated the Astoria and Flushing lines in Queens (today's 7 Train and the Queens part of the N Train). For this project the IRT was given use of the Steinway tunnels, tunnels that connected Manhattan and Long Island City that were built in the 19th century. The IRT was now in four boroughs.

In the late 20s and early 30s, the city, under mayor John Hyland, a disgruntled former employee of the BRT, embarked on a project to build and operate its own subway lines, competing directly with the BMT and IRT in the hopes of driving them to bankruptcy and allowing the city to assume control of all transit operations. The Independent, or IND, as the city owned lines were called, took business from the IRT by building a subway under Sixth Avenue, where there was already an IRT elevated, as well as building the Concourse line a block away from an IRT elevated line in the Bronx. As the IRT and BMT were built to different width and tunnel clearance specifications, the city chose to build the IND to BMT standards to facilitate the eventual integration of those two systems. The legacy of this today is that the lines of the former IRT feature train cars that are a foot narrower than the lines of the other two systems, a difference that is really noticeable in terms of crowding and claustrophobia.

Finally, in 1940, the BMT and IRT were willing to sell and the city assumed all transit operations. The new city transit agency went about consolidating the various lines, and making both track and transfer connections between the formerly separate systems. Although direct service connections between the rest of the system and the IRT were impossible because of different the train specifications, many of the stations were connected with free transfers, integrating the former IRT into the rest. The city tore down all of the former IRT elevateds in Manhattan, including the Second, Third, Sixth, and Ninth avenue lines. Trackage of the former New York, Westchester and Boston railway in the Bronx was incorporated into the IRT system, forming part of today's 5 Train. The IRT lines were assigned numbers in the new transit line naming scheme. The three system (BMT, IRT, and IND) distinction remained on maps though, until 1967, when the Chrystie Street connection from the Manhattan Bridge to the Sixth avenue line was completed and the lines of the IND and BMT were sufficiently intermixed to make division irrelevant. Currently the Metropolitan Transportation Authority operates all of the lines of the New York City subway.

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