When the Nostrand Avenue Line is extended southwards, one platform of the Flatbush Avenue station would be closed at a time to allow the station to be renovated and the platforms to be extended north by about 100 feet. While the platforms are being extended, a new crossover would be constructed midway between the station and Newkirk Avenue station (with a new crossover to be constructed south of Church Avenue station as well); this would require at least twelve consecutive weekend shutdowns of the entire line. The southern entrance, the southernmost 100 feet of platforms, and the bumper posts would then have to be demolished, and a temporary wall and walkway would be erected at that spot. Just south of Avenue H, two giant TBM launch boxes would be created. A 500 foot long cut-and-cover section under Nostrand would have to be constructed in order to connect the new extension with the existing line. During the construction project, normal service would be run on weekdays. However, on weekends, all train services would terminate at Church Avenue, and 5 trains would have to be cut back to Bowling Green.
Four alternatives would be studied for the extension:
1. Build a fully underground extension of the Nostrand Avenue Line from Flatbush Avenue station to East Sheepshead Bay. New stations would be located at the following streets: Avenue K, Kings Highway, Quentin Road, Avenue T, and Avenue X. Utica Avenue Line would be built from Crown Heights-Utica Avenue to Avenue U-Kings Plaza. New underground stations would be built at: Empire Boulevard-Remsen Avenue, Clarkson Avenue-Lenox Road, Snyder Avenue, Foster Avenue, Kings Highway-Avenue H, Flatlands Avenue, and Avenue N. After the Avenue N station, a tunnel portal would be built just south of Fillmore Avenue to carry the line on a concrete elevated viaduct over Flatbush Avenue to the terminus at Avenue U-Kings Plaza.
2. Build a fully underground extension of the Nostrand Avenue Line from Flatbush Avenue station to Kings Highway station, then build a fully underground extension to Gerritsen Beach. New stations would be built under Nostrand Avenue at Avenue K and Kings Highway. After Kings Highway station, the line shall be diverted under Gerritsen Avenue and new stations would be built at the following locations: Fillmore Avenue-Marine Park, Avenue U-Knapp Street, and Gerritsen Beach-Florence Avenue. The Utica Avenue Line shall be built as in proposal 1.
3. Build a fully underground extension of the Nostrand Avenue Line from Flatbush Avenue station to Kings Highway station, then build an extension to Bergen Beach. The Nostrand Avenue alignment would be followed to Kings Highway. After Kings Highway station, a new station would be built under Gerritsen Avenue at Avenue R, then the line would be built under Filmore Avenue with a new station at East 36th Street. The line would continue under Filmore Avenue with a station at East 53rd Street and then under Avenue T to new stations at Mill Avenue and Veterans Avenue/East 69th Street, where the line would terminate. If this extension is constructed, then the Utica Avenue Line would not be built.
4. Build an extension of the Nostrand Avenue Line under Flatbush Avenue. Stations would be located at Avenue J, Kings Highway-Flatlands Avenue, and Avenue R. After Avenue R, the line would run to a tunnel portal just south of Filmore Avenue, where the line would continue on a concrete elevated viaduct to Avenue U-Kings Plaza. A streetcar line would be built on Nostrand Avenue between Williamsburg and East Sheepshead Bay; the streetcar line would stop at the same stations as the B44 SBS. In addition, the Utica Avenue Line would be built as a streetcar line; it would stop at the same stations as the B46 SBS.