The Sea Beach Line
The BMT Sea Beach Line (N train) also used to terminate at the 64th Street Ferry pier and when the 4th Avenue BMT (N/R train) was built in 1915, the interconnection of the two lines allowed trains coming from the north to fork at 66th street (just east of the Bay Ridge rail yard) with the R train proceeding south into Bay Ridge proper and the N train headed east and then south to Coney Island.
The BMT Sea Beach Line shares the rail ROW (on separate tracks) with the Bay Ridge Line between 4th Avenue and 14th Avenue. Presumably, the Circumferential Line would just use the existing BMT tracks for this portion of its run to save construction costs, although there might be some congestion issues at rush hour.
The streets to the north of the tracks are largely devoted to commercial activity, much of it by Chinese businesses.
Rail ROW viewed from 5th Ave. bridge looking east - BMT subway on the left, freight rail line on the right
Rail ROW viewed from 5th Ave. bridge
Fifth Avenue retail district just north of rail line
China Perfect Iron and Stainless - 525 63rd St.
Bay Ridge Live Poultry - 551 63rd St.
Beit El-Maqdis Islamic Center - 6224 Sixth Ave.
Beit El-Maqdis Islamic Center
Beit El-Maqdis - ladies entrance
Beit El-Maqdis Islamic Center tower
MTA signal house at Sixth Ave.
Sixth Ave. bike lane
Rail lines proceed east from Sixth Ave.
K.K. Live Poultry? - 6221 Seventh Ave.
Ming Gee Seafood Palace
Fiber optic sign
Commercial space on 61th Street between 6th and 7th Avenues
Where bamboo shoots come from
Storage rooms on 61th Street