Built for the Interborough Rapid Transit Company (IRT), it stretches from Downtown Brooklyn south along Flatbush Avenue and east along Eastern Parkway to Crown Heights.

The west end of the Eastern Parkway Line is at the Joralemon Street Tunnel under the East River.

Contract 2 extended the original line from City Hall in Manhattan to Atlantic Avenue in Brooklyn.

[33] Under the revised plan, the four-track tunnel under Eastern Parkway was to be double-decked, except at the Franklin Avenue station, where all tracks would be on the same level.

[34] In April 1915, nineteen companies submitted bids to construct the section of line between Grand Army Plaza and Nostrand Avenue.

[40][41] The tunnel between Grand Army Plaza and Nostrand Avenue was built using the cut-and-cover method, with two steam shovels excavating an estimated 600,000 cubic yards (460,000 m3).

[43] Mayor John Francis Hylan inspected the line on August 20, 1920, prior to its official opening.

[62] The opening of the tunnel allowed access to Brooklyn via the IRT from both the East and West Sides of Manhattan.

[64] In August 1961, the chairman of the New York City Transit Authority, Charles Patterson, announced a $2.5 million project that would get rid of a trouble spot on the line between Nevins Street and Atlantic Avenue that slows service and backs up the IRT Division.

[66] The MTA announced in October 2020 that it would renovate the Eastern Parkway Line tunnels between Borough Hall and Franklin Avenue.

[67][68] The renovations were announced following two incidents in 2018, when ceilings at the Borough Hall and Atlantic Avenue stations partially collapsed, injuring passengers.

The first station along this segment is Nevins Street, which contains a never used lower level, and then joins Atlantic Avenue–Barclays Center, the end of the oldest section of the line.

[2] East of Grand Army Plaza, the line finally moves under its namesake, the first station serving the Brooklyn Museum.

Afterwards, the IRT Eastern Parkway Line ends under Ralph Avenue, one block east of its originally intended terminus, whereas the local tracks become the IRT New Lots Line, branching off to the southeast emerging from the ground near Buffalo Avenue at Lincoln Terrace Park.