The M service has historically served the Nassau Street Line, but since 2010, the M has been rerouted via the Chrystie Street Connection to run on the IND Sixth Avenue Line, as a replacement for the V, which was discontinued due to financial shortfalls.

Trains coming from Brooklyn via the Manhattan and Williamsburg Bridges would be able to head back to that borough via the Brooklyn Bridge as well as the Montague Street Tunnel at the south end of the Centre Street Loop, and vice versa.

All trains would pass through a large central station with four tracks and five platforms at Chambers Street, just north of the Brooklyn Bridge.

[7] The city began receiving bids for the sections under Centre Street in March 1907.

[9][10] The line was assigned to a proposed Tri-borough system in early 1908 and to the Brooklyn Rapid Transit Company (BRT) in the Dual Contracts, adopted on March 4, 1913.

[18] In March 1913, the Public Service Commission authorized the BRT to lay tracks, install signals, and operate the loop.

[22] The south tracks on the Manhattan Bridge, also running into Chambers Street, were placed in service on June 22, 1915.

[27] However, mayor John Hylan refused to act during his final two years in office.

[29] By January 1925, the BMT was asking its passengers to pressure Hylan into approving the remainder of the Nassau Street Line.

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.

[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.

The new line provided an additional ten percent capacity compared with existing service through DeKalb Avenue.

[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.

This ended all "loop" service, which had most recently seen rush hour "specials" on both the Brighton and 4th Avenue lines operating via both the Manhattan Bridge and Montague Street tunnel in single directions.

[57][58][59] In 2024, as part of a program to upgrade the signaling of the New York City Subway, the MTA proposed installing communications-based train control (CBTC) on the Nassau Street Line as part of its 2025–2029 Capital Program.