[10]: 3  In 1832, the New York State Legislature authorized the company to raise its stock limit to $500,000 on the condition that the line be completed to the Harlem River by 1835.

[10]: 4 [11] On April 27, 1837, an act was passed in the State Legislature to widen Fourth Avenue between 32nd Street and the Harlem River to provide room for the railroad.

[10]: 6  In that same year, the railroad purchased six city lots at Fourth Avenue and 26th Street for $7,000, for the construction of car barns and stables.

Work on the Murray Hill rock cut began in autumn, 1833 and was completed in 1834, and service along a 4.432 miles (7.133 km)-long segment to 85th Street in Yorkville opened on May 1, 1834.

[9]: 27 [19] Since there were originally no grade-separated crossings of the railroads between 42nd and 59th Streets,[20] there were frequent accidents; seven people died within 12 days of the move to Grand Central.

The confluence of tracks to the north of Grand Central was considered to be the city's "most fearful death-trap" by The New York Times in 1872, and large meetings were held to protest the deaths caused by collisions between trains and pedestrians.

The New York State Legislature passed legislation in 1872, requiring that 4+1⁄4 miles (7 km) of NY&H tracks between Grand Central and the Harlem River be placed underground.

[23] In June, 1874, New York City Mayor William Frederick Havemeyer refused to sign warrants in payment for the project, as he believed that the Legislature's act was unconstitutional.

On June 20, the entire improvement opened, and the first trains from Grand Central to the Harlem River were witnessed by large crowds of spectators.

[41] In January, 1876, a test case went on trial in New York Supreme Court, brought by Charles Higham against the NY&H and a contractor for injuries to his business that resulted from obstructions raised by construction on the project.

[45] On May 15, 1876, partial rapid transit began on the Harlem Line, with sixteen trains a day running between Grand Central Depot and William's Bridge.

[48] Loud daily complaints were made to William H. Vanderbilt, President of the NY&H, urging him to equip these two stations on the line, half of which was paid for by the city's taxpayers.

While Cornelius Vanderbilt had originally indicated his intent to continue the underground line to City Hall, there was speculation that he did not intend to build the section south of 42nd Street.

[52] William Henry Vanderbilt stated that the line would not be as dark as the Metropolitan Railway (now part of the London Underground), and that there would be stations every eight blocks, or every .4 miles (0.64 km).

[29] According to a prominent officer of the New York Central, a member of the Board of Directors, Commodore Cornelius Vanderbilt had fully intended to extend the underground steam railroad to City Hall, and had plans drawn and ready to be executed.

His son, William H. Vanderbilt, objected to the plan after returning from London, and, having ridden on the Metropolitan Railway, believed that the public would not tolerate riding in a dark tunnel.

[54] Commodore Vanderbilt decided not to build the line due to public criticism for the grant, opposition to the project[50]: 104  from business people and homeowners in the Bowery, and because of the Panic of 1873.

The New York Central was opposed to the project as the increase in river traffic would interfere with its rail line, which was only 8 feet (2.4 m) above the water.

[86] The New York state legislature subsequently passed a law to ban all steam trains in Manhattan beginning on July 1, 1908.

The first electric train departed for the soon-to-be-demolished Grand Central Depot from the Harlem Division's Highbridge station in the Bronx, on September 30, 1906.

[92] With the elimination of the noise and pollution from steam locomotives, this once-undesirable section of Park Avenue and the land freed by the covering-over of the rail yard became prime New York City real estate.

[97] On September 11, 1931, Manhattan Borough President Samuel Levy proposed placing the elevated portion of the line between 96th Street and the Harlem River in a tunnel to reduce train delays and improve property values.

In response to the tax hike, New York Central President Alfred E. Perlman stated that the city was trying to run the company out of town by making its operations unprofitable.

[106] As a result of the massive increase, on September 10, 1956, the railroad applied with the New York State Public Service Commission for permission to raise commuter fares for the first time in four years.

[108] In 1975, work began on a project to replace a 350 foot (110 m)-long steel viaduct, which carries the four-track line between the Harlem River Vertical Lift Bridge and Mott Haven Interlocking and over 138th Street at the Grand Concourse.

An amendment to the Metropolitan Transportation Authority (MTA)'s 1982–1986 Capital Program in 1983 provided $46 million in additional funding for the project.

As part of the project, structural steel, aged wiring conduits and the roadbed were replaced; the side walls and waterproofing were repaired; and a new drainage system was installed.

Other work completed as part of the project included the modernization of the signal system and the construction of new entrances to the north of Grand Central Terminal.

Due to the high cost of making these upgrades, and the lack of cooperation between the New York City Department of Transportation and Metro-North, which had opposed any solution that would modify the tunnel roof, the project was delayed for several years.

In July, the MTA Board approved the allocation of $3.3 million to pay for the construction of temporary supports for the structure, which would eliminate speed restrictions imposed for diesel locomotives.