It called for a subway line from New York City Hall in lower Manhattan to the Upper West Side, where two branches would lead north into the Bronx.

[14]: 168  As part of a modification to the IRT's construction contracts made on January 18, 1910, the company was to lengthen station platforms to accommodate ten-car express and six-car local trains.

An elevator shaft would have connected the Steinway Tunnel, a platform with the Hudson & Manhattan Railroad (H&M, now PATH), and the new concourse, and would have led out to street level adjacent to a stairway leading to an extension of the IRT Third Avenue Line.

[34] The commission voted in favor of the original diagonal route in February 1914,[35] at which point the Grand Union Hotel was condemned via eminent domain.

[44] In March 1916, the Public Service Commission authorized the IRT to build a new mezzanine passage at the station to reduce platform crowding for $6,000 (equivalent to $168,000 in 2023).

The concourse for the station would have been located on the north side of 42nd Street between Depew Place and Park Avenue, with stairways connecting to the Steinway Tunnel platform below.

[58] A franchise to extend the Uptown Tubes to Grand Central was awarded in June 1909, with the expectation that construction could start within six months and that the new extension would be ready by January 1911.

[61] This time, the Rapid Transit Commissioners declined this request for a delay, effectively ending the H&M's right to build an extension to Grand Central.

[62] In August 1925, Eastern Offices Inc. signed an agreement to lease land from the New York Central for 21 years to construct the Graybar Building.

[66] In 1928, to alleviate overcrowding on the Lexington Avenue Line, a consulting engineer for the New York State Transit Commission proposed the construction of "reservoir" stations at 33rd/34th and 42nd Streets.

[67] Joseph V. McKee wrote a letter to the IRT the next year, saying that overcrowding at the station during rush hours created life-threatening conditions.

[73] The IRT sued to block construction of the new entrance because it would cause crowding,[74] but the New York City Board of Transportation pushed to allow the corridor anyway.

[88] As part of a pilot program, the New York City Board of Transportation installed three-dimensional advertisements at the Grand Central station in late 1948.

[92]: 39 In April 1954, the Bowery Savings Bank completed the installation of a two-speed, reversible escalator from the ground floor of the building from the south side of 42nd Street between Pershing Square and Lexington Avenue to the station mezzanine.

The most expensive element of the plan was the construction of a lower level station with multiple platforms and storage tracks,[97] which was expected to cost $20 million (equivalent to $226,914,000 in 2023).

[92]: 29–30 On March 10, 1955, the New York City Transit Authority (NYCTA) awarded a $1.6 million (equivalent to $18,198,000 in 2023) engineering contract for design, inspection, and field supervision for the project to Parsons, Brinckerhoff, Hall and MacDonald.

[99] The NYCTA announced plans in 1956 to add fluorescent lights to the Flushing Line and shuttle platforms at the Grand Central station.

[102] On March 17, 1964, construction began on a $1 million (equivalent to $9,824,000 in 2023) project to replace three elevators serving the Flushing Line platform with two sets of 4 foot (1.2 m)-wide escalators, on two levels leading to the station mezzanine.

It qualified for the program due to a significant investment in the area by private developers, including the rebuilding of the Commodore Hotel as the Grand Hyatt, the renovation of the Chrysler Building, and the construction of a new headquarters for Philip Morris.

The passageway had been located behind a token booth, making it hard to patrol; at the time of its closure, the hallway was described as being "deceptively long and treacherous".

The project, which was financed using state and Federal funds and designed by Gruzen Samton Architects, would focus on improving the appearance of the station, and would be constructed in phases.

[123] In 2015, SL Green gave $220 million (equivalent to $282,791,000 in 2023) toward the building's construction, of which two-thirds of the money would be used for station redesign;[126][127] this marked the largest private investment to date to the New York City Subway system.

[128][129] As part of the 2015–2019 MTA Capital Program, the 42nd Street Shuttle became ADA accessible, the center track was removed, and the trains became six cars long.

[138] A new mezzanine below the existing mezzanine, known as the Short Loop connection, has been proposed to provide a direct connection from the subway station to the Grand Central Terminal's lower-level Metro-North platforms, and to the concourse of the Long Island Rail Road's Grand Central Madison station, the latter of which was built as part of the East Side Access project.

[140][141] Further circulation improvements are planned as part of a replacement of the Hyatt with a skyscraper at 175 Park Avenue, to be called Project Commodore, which is expected to be built from 2022 to 2030.

An east-west passageway connects the Lexington Avenue Line's mezzanine to the 42nd Street Shuttle, which also has its own dedicated entrance and exit stairs.

[155] In 2000, the Lexington Avenue Line station received air conditioning after Metro-North Railroad installed chillers for Grand Central Terminal.

From the public passageway, none of the original support columns and roof are visible, since they were removed in exactly this area to open the way for the unused shuttle station.

After the merge, the pairs of tracks in each direction diverge, with two on each side of the 1870 New York and Harlem Railroad Murray Hill Tunnel, which is now used for automobile traffic on Park Avenue.

Two sections of the old Steinway Tunnel loop remain intact and are accessible to MTA personnel via the southbound track approximately 200 feet (61 m) beyond the station.