Question: In what neighborhood of Upper Manhattan, New York City, does the Second Avenue Line of the New York City Subway border?

Context paragraphs:
Second Avenue (IND Sixth Avenue Line): Second Avenue is a station on the IND Sixth Avenue Line of the New York City Subway, located at the intersection of Second Avenue and Houston Street on the border between the East Village and the Lower East Side, in Manhattan.  It is served by the F train at all times.  Limited M train service also stops here during rush hours.

East Harlem: East Harlem, also known as Spanish Harlem or El Barrio, is a neighborhood of Upper Manhattan, New York City roughly encompassing the area north of the Upper East Side and East 96th Street up to about the 140s, east of Fifth Avenue to the East and Harlem Rivers.  It lies within Manhattan Community District 11.  Despite its name, it is generally not considered to be a part of Harlem.

72nd Street (Second Avenue Subway): 72nd Street is a station on the first phase of the Second Avenue Line of the New York City Subway.  Located at the intersection of Second Avenue and 72nd Street on the Upper East Side, it opened on January 1, 2017, with the Q train providing full-time service, as well as some N service during rush hours.  This is the southernmost station on the first phase of the Second Avenue Line; south of this station, the BMT 63rd Street Line diverges to the west, towards the Lexington Avenue–63rd Street station, and bellmouths exist for a future extension to Second Avenue–Houston Street and Hanover Square.

116th Street (Second Avenue Subway): 116th Street is a planned station along the IND Second Avenue Line of the New York City Subway.  It would be located at the intersection of Second Avenue and 116th Street in East Harlem, Manhattan.  Proposed since 1968, the station is expected to be built by 2027–2029 as part of Phase 2 of the Second Avenue Subway.  When opened, it will initially be served by the Q train, with the T providing service when Phase 3 of the line is built.

R9 (New York City Subway car): The R9 was a New York City Subway car which was built in 1940 for the IND and its successors, which included the New York City Board of Transportation and the New York City Transit Authority.  A total of 153 R9 (or "Arnine") cars were ordered from two different manufacturers.  Cars 1650-1701 were built by American Car and Foundry, while cars 1702-1802 were built by Pressed Steel.  150 of the new cars were ordered for service on the new IND Sixth Avenue Line, which opened on December 15, 1940.  The Sixth Avenue Line was the second IND Manhattan trunk line (joining the 1932 IND Eighth Avenue Line), and therefore the additional cars were needed for the new service.  The remaining 3 cars in the R9 contract were ordered as replacements for 3 older IND cars (cars #212, #378, and #472) that had been damaged beyond repair as a result of a February 17, 1936 collision at the Smith–Ninth Street station.

106th Street (Second Avenue Subway): 106th Street is a planned station along the IND Second Avenue Line of the New York City Subway.  It would be located at the intersection of Second Avenue and 106th Street in East Harlem, Manhattan.  Proposed since 1968, the station is expected to be built by 2027–2029 as part of Phase 2 of the Second Avenue Subway.  When opened, it will initially be served by the Q train, with the T providing service when Phase 3 of the line is built.

Second Avenue Subway: The Second Avenue Subway (officially the IND Second Avenue Line; abbreviated to SAS) is a New York City Subway line that runs under Second Avenue on the East Side of Manhattan.  The first phase of this new line opened on January 1, 2017, connecting three new stations between 96th Street and the 63rd Street Lines to the BMT Broadway Line and the rest of the subway system.  The full Second Avenue Line, if and when funded, is to be built in three more phases that will eventually connect 125th Street to Hanover Square.  The proposed full line would be 8.5 mi long with 16 stations and a projected daily ridership of 560,000, costing more than $17 billion.

History of the Second Avenue Subway: The Second Avenue Subway, a New York City Subway line that runs under Second Avenue on the East Side of Manhattan, has been proposed since 1919.  The first phase of the line, consisting of three stations on the Upper East Side, started construction in 2007 and opened in 2017.  From 1919 to the 1960s, many distinct plans for the Second Avenue subway line were never carried out, though small segments were built in the 1970s.  The complex reasons for these delays are why the line is sometimes called "the line that time forgot".

129th Street (IRT Third Avenue Line): 129th Street was a massive station on the IRT Third Avenue Line, shared by trains of both the Third Avenue Line and IRT Second Avenue Line in the New York City Subway system.  The next stop to the north was 133rd Street for the main line and Willis Avenue for the Willis Avenue spur, both of which were across a swing bridge above the Harlem River in The Bronx.  The next stop to the south was 125th Street−Third Avenue for Third Avenue Line trains and 125th Street−Second Avenue for Second Avenue Line trains.

96th Street (Second Avenue Subway): 96th Street is a station on the IND Second Avenue Line of the New York City Subway.  Located at the intersection of Second Avenue and 96th Street on the border of the Upper East Side and East Harlem neighborhoods in Manhattan, it is the northern terminus for the Q train at all times, and for some N trains during rush hours.  This station is the terminus for the first phase of the Second Avenue Line.

Supporting paragraphs: 96th Street (Second Avenue Subway) East Harlem
Answer: East Harlem