The first regularly operated line of the New York City Subway was opened on October 27, 1904, and was operated by the Interborough Rapid Transit Company (IRT). The early IRT system consisted of a single trunk line running south from 96th Street in Manhattan (running under Broadway, 42nd Street, Park Avenue, and Lafayette Street), with a southern branch to Brooklyn. North of 96th Street, the line had three northern branches in Upper Manhattan and the Bronx. The system had four tracks between Brooklyn Bridge–City Hall and 96th Street, allowing for local and express service. The original line and early extensions consisted of:
- The IRT Eastern Parkway Line from Atlantic Avenue–Barclays Center to Borough Hall
- The IRT Lexington Avenue Line from Borough Hall to Grand Central–42nd Street
- The IRT 42nd Street Shuttle from Grand Central–42nd Street to Times Square
- The IRT Broadway–Seventh Avenue Line from Times Square to Van Cortlandt Park–242nd Street
- The IRT Lenox Avenue Line from 96th Street to 145th Street
- The IRT White Plains Road Line from 142nd Street Junction to 180th Street–Bronx Park
Planning for a rapid transit line in New York City started in 1894 with the enactment of the Rapid Transit Act. The plans were drawn up by a team of engineers led by William Barclay Parsons, chief engineer of the Rapid Transit Commission. The city government started construction on the first IRT subway in 1900, leasing it to the IRT for operation under Contracts 1 and 2. After the initial line was opened, several modifications and extensions were made in the 1900s and 1910s.
The designs of the underground stations are inspired by those of the Paris Métro; with few exceptions, Parsons's team designed two types of stations for Contracts 1 and 2. Many stations were built just below or above street level, as Parsons wished to avoid using escalators and elevators as the primary means of access to the station. Heins & LaFarge designed elaborate decorative elements for the early system, which varied considerably between each station, and they were also responsible for each station's exits and entrances. Most tunnels used cut-and-cover construction, although deep-level tubes were used in parts of the system; elevated structures were used in Upper Manhattan and the Bronx. The lines used third rail power supplied by the IRT Powerhouse, as well as rolling stock made of steel or of steel–wood composite.
The city could only afford one subway line in 1900 and had hoped that the IRT would serve mainly to relieve overcrowding on the existing transit system, but the line was extremely popular, accommodating 1.2 million riders a day by 1914. Although the subway had little impact on retail in Lower and Midtown Manhattan, the completion of the IRT subway helped encourage other development, including residential growth in outlying areas and the relocation of Manhattan's Theater District. The Dual Contracts, signed in 1913, provided for the expansion of the subway system; as part of the Dual Contracts, a new H-shaped system was placed in service in 1918, splitting the original line into several segments. Most of the original IRT continues to operate as part of the New York City Subway, but several stations have been closed.