Ditmars Boulevard Station
The BMT Astoria Line terminates underneath an NYCCRR arch bridge over 31st Street. This would be a likely spot for a transfer point between the Circumferential Line and the BMT Astoria. If there's space on the deck for platforms, escalators and elevators would either have to descend inside the masonry or, more likely, along the outside of the arch.
Next: Hell Gate Bridge