This chaotic melange of tracks was originally the point where the Brodway and Fulton Elevated lines met in 1884 (remnants are the J/Z trains). The BMT Canarsie line (L Train) joined the fray in 1928 and the subterranean IND Fulton Street Line (A/C trains) in 1946. Adding to the mess is a the East New York Subway Yard just to the east, although the trolly lines that ran on the streets below are now replaced by buses. The complex has had numerous modifications and renovations over the years, sprinkling in some abandoned sections and vestigial ornamentation.
Underneath all this somewhere is the Bay Ridge Line East New York Tunnel. There was not a station here during the line's passenger days and I'm not sure if there is any kind of connection (or room for platforms) between the IND tunnel and the Bay Ridge Line tunnel. The Circumferential Line and the Canarsie BMT would probably be sharing tracks here. If the line would keep the high BMT platform, limited modifications would be needed. If they were both placed in the Bay Ridge Line tunnel, providing platforms and a transfer corridor would likely be a major task.
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