1920s

On June 27, 1922, the New York State Transit Commission commissioned its engineers to examine platform-lengthening plans for 23 stations on the lines of the Brooklyn–Manhattan Transit Corporation (BMT), the successor to the BRT, to accommodate eight-car trains. As part of the project, Union Street's platforms would have been lengthened from 435 feet (133 m) to 530 feet (160 m).^[13][14] Though the Transit Commission ordered the BMT to lengthen these platforms in September 1923, no further progress was made until February 16, 1925, when the New York City Board of Transportation (NYCBOT) commissioned its engineers to examine platform-lengthening plans for this and eleven other stations along the Fourth Avenue Line. It estimated the project would cost $633,000 (equivalent to $10,998,000 in 2023).^[15] The NYCBOT received bids for the project on February 25, 1926.^[16] The contract was awarded to the Corson Construction Company for $345,021 (equivalent to $5,938,000 in 2023).^[17] The extensions opened on August 1, 1927.^[6]

1950s and 1960s

The city government took over the BMT's operations on June 1, 1940.^[18][19] In July 1959, the New York City Transit Authority (NYCTA) announced that it would install fluorescent lighting at the Union Street station and five other stations along the Fourth Avenue Line for between $175,000 and $200,000. Bids on the project were to be advertised on August 7, 1959, and completed by fall 1960.^[20]

In the 1960s, the NYCTA started a project to lengthen station platforms on its lines in Southern Brooklyn to 615 feet (187 m) to accommodate 10-car trains.^[7] On July 14, 1967, the NYCTA awarded a contract to conduct test borings at eleven stations on the Fourth Avenue Line, including Union Street, to the W. M. Walsh Corporation for $6,585 (equivalent to $60,172 in 2023) in preparation of the construction of platform extensions.^[21] The NYCTA issued an invitation for bids on the project to extend the platforms at stations along the Fourth Avenue Line between Pacific Street and 36th Street, including those at Union Street, on March 28, 1969.^[22] The contract was awarded to the Horn Construction Company on May 13, 1969, for $7,381,890 (equivalent to $61,333,000 in 2023). Funding for the renovation projects came out of the NYCTA's 1969–1970 Capital Budget, costing $8,177,890 (equivalent to $67,946,000 in 2023) in total.^[23]

As part of the renovation project, the station's platforms were extended 85 feet (26 m)^[7] to the north,^[24] and the station's elaborate mosaic tile walls were covered over with 8-by-16-inch (20 by 41 cm) white cinderblock tiles. The latter change, which was also made to 15 other stations on the BMT Broadway and Fourth Avenue Line, was criticized for being dehumanizing. The NYCTA spokesman stated that the old tiles were in poor condition and that the change was made to improve the appearance of stations and provide uniformity. Furthermore, it did not consider the old mosaics to have "any great artistic merit".^[25]

1990s

The station was renovated again from 1991 to 1994. The $2.397 million (equivalent to $5,362,000 in 2023) contract for the project was awarded on February 28, 1991. During the closure of the Bay Ridge-bound platform, the glazed tiles at the station were heavily defaced with graffiti, and were cleaned prior to the station's reopening with a heavy duty cleaner, which damaged the glazing. A modification to the contract was approved on May 23, 1994. As part of the modification, new graffiti resistant ceramic tile was installed, signage, advertising panels, benches and trash bins were removed and reinstalled, exhaust systems were added in the porter rooms at both platforms, and the wall-mounted conductors board was replaced by a ceiling-mounted one. The modification cost $174,000 (equivalent to $358,000 in 2023), and change orders cost $111,004 (equivalent to $228,000 in 2023).^{[26]: C-99, C-100} The Brooklyn-bound platform was closed between July 15 and November 15, 1991,^[27] while the Manhattan-bound platform was closed between February 19 and June 20, 1992.^[28][29]

In addition to upgrading the same elements that were replaced in the previous overhaul, floors and track wall tiling, the public announcement system, and safety treads along platform edges and track-beds were replaced. It also included an art installation by Emmett Wigglesworth called CommUnion. It features twenty-two panels of various designs in the recessed area of the platform tiles above the station signs and other designs on the openings in the track walls.^[30][31]

Proposed accessibility

As part of the 2015–2019 Metropolitan Transportation Authority Capital Program, $300 million was allocated to enhance station access and provide ADA-accessibility at 15 stations chosen by New York City. Four stations were being evaluated for such improvements, including the Union Street station.^[32][33]

In 2019, development firm Avery Hall Investments offered to pay $11 million for a new entrance to the southbound platform, including an elevator, as part of the development of a 17-story building on the northwestern corner of Fourth Avenue and Union Street (651 Union Street).^[34][35][36] In exchange, the developers would be able to add 50,000 square feet (4,600 m²) of space to their building.^[35][36] In late March 2022, 204 4th Avenue LLC submitted an application for the new easement entrance to the New York City government.^[37]