Construction

Under Contract 4 of the Dual Contracts, the BRT (later reorganized as the Brooklyn–Manhattan Transit Corporation or BMT) was to operate the Nassau Street Line.^[24] The southern portion of the line remained incomplete for several years, and the BRT brought a $30 million suit against the city for not building the line before January 1, 1917.^[25] Most of the BMT's Dual Contracts lines were completed by 1924, except for the Nassau Street Line.^[24] BMT chairman Gerhard Dahl was persistent at requesting that the city build the line, saying in 1923 that the BMT was willing to operate the line as soon as the city completed it.^[26] At the time, the BMT was planning to construct two stations on the Nassau Street Line: one at Fulton Street, where the BMT planned to place the northbound platform above the southbound platform due to the street's narrowness, and the other at the intersection at Broad and Wall Streets, where both platforms would be on the same level.^[27] However, mayor John Hylan refused to act during his final two years in office.^[24] BOT chairman John H. Delaney believed that the line was unnecessary because both of its planned stations would be extremely close to existing subway stations.^[28] Meanwhile, the BMT claimed that the city's failure to complete the line was overburdening other BMT lines.^[29] By January 1925, the BMT was asking its passengers to pressure Hylan into approving the remainder of the Nassau Street Line.^[30] Work did not commence until after James Walker succeeded Hylan as mayor at the end of 1925.^[24]

The city government agreed to build the Nassau Street Line in May 1927,^[31] after the BMT sued the city for $30 million.^[31][32] At the time, the city wanted to take over the BMT's lines but could not do so until all Dual Contracts lines were completed.^[32] The BOT received bids for the construction of the line that July,^[33] but it rejected every bid the next month because of concerns over the lowest bidder's ability to complete the work.^[32][34] That September, contractors again submitted bids to the BOT; some bidders offered to build the entire line, while others only offered to construct the segments of the line to the north or south of Liberty Street.^[33] The BOT awarded construction contracts for the line's construction two months later. The Marcus Contracting Company was hired to build the portion north of Liberty Street, including the Fulton Street station, for $4.7 million, while Moranti and Raymond were hired to build the portion to the south for $5.7 million. ^[35][36] The New York City Board of Estimate approved the contracts in January 1928, allowing the builders to construct the line using the cut-and-cover method, despite merchants' requests that the line be constructed using tunnelling shields.^[37]

The line was constructed 20 feet (6.1 m) below the active IRT Lexington Avenue Line, next to buildings along the narrow Nassau Street, and the project encountered difficulties such as quicksand.^[24][38] When the construction contracts were awarded, work had been projected to be completed in 39 months.^[24] By early 1929, sixty percent of the work had been finished.^[24][39] Nassau Street is only 34 feet (10 m) wide, and the subway floor was only 20 feet (6.1 m) below building foundations. As a result, 89 buildings had to be underpinned to ensure that they would stay on their foundations. Construction had to be done 20 feet below the active IRT Lexington Avenue Line. An area filled with quicksand with water, which used to belong to a spring, was found between John Street and Broad Street. Construction was done at night so as to not disturb workers in the Financial District.^[24] The project was 80 percent complete by April 1930,^[40] and Charles Meads & Co. was awarded a $252,000 contract to install the Fulton Street station's finishes the next month.^[41][42] The plans for that station had been changed so that the southbound platform was above the northbound platform.^[41][42] The total construction cost was $10.072 million^[38][43] for 0.9 miles (1.4 km) of new tunnels, or $2,068 per foot ($6,780/m), which was three times the normal cost of construction at the time.^[24]

Opening of loop

The Nassau Street Loop opened at 3 p.m. on May 29, 1931, when Mayor Jimmy Walker took the controls of a train of D-type Triplex cars from Chambers Street to Broad Street.^[25][44][45] This completed what was known as the Nassau Street Loop.^[25] The loop ran from the line's previous terminus at Chambers Street, running through the Fulton Street and Broad Street stations before merging with the Montague Street Tunnel to Brooklyn.^[46] The completion of the line relieved congestion on several BMT lines to southern Brooklyn, which previously had to operate to Midtown Manhattan using the Broadway Line.^[47] When the line was completed, Culver Line trains began operating on the loop; previously, elevated Culver Line trains from Coney Island ran only as far as Ninth Avenue, where transfers were made to West End subway trains. The new line provided an additional ten percent capacity compared with existing service through DeKalb Avenue. Service on the Jamaica Line was extended to operate to this station.^[48] The station at Wall Street was named "Broad Street" to distinguish it from the already-open Wall Street stations on the Lexington Avenue Line and Seventh Avenue Line.^[38]

Plans for the Chambers Street area changed several times during construction, always including a never-completed connection to the Brooklyn Bridge tracks. By 1910, only the west two tracks were to rise onto the bridge, and the east two were to continue south to the Montague Street Tunnel. As actually built for the 1931 opening south of Chambers Street, the two outer tracks ran south to the tunnel, while the two inner tracks continued several blocks in a lower level stub tunnel to allow trains to reverse direction.^[49]