Anything north of Bay was fill created with debris from 1906. Columbus Avenue followed a stream bed. Common knowledge available to ordinary folk like me.

Relocating utilities and other tasks is cheaper on one street rather than two. Thus a "simple" one street subway rather than on two streets.

Now engineering a grade separated wye north of Filbert & Columbus that is going to take some civil engineering expertise, way beyond my pay grade. If I were Muni I would plan instead on connecting Fillmore & Lombard(putative center of the Marina) to a line on Geary - Pacific Hts. is not fill. Straightforward and direct and roughly parallels the existing #30 express bus.