Chinatown area, the article says they walked for a bit from DUMBO which would make sense to cross the bridge. http://secondavenuesagas.com/2013/06/25/abandoned-sas-segmen...

One guess--if it was in Brooklyn and was a station with no tracks laid down (so never actually used as a station)--is that it was part of the IND second line. Maybe the South 4th Street and Broadway station?

This line was started just before the 1929 crash and never completed. See http://www.columbia.edu/~brennan/abandoned/indsecsys.html, there's a hand-drawn map down at the bottom.

The author said they started at a DUMBO station. Then they walked for half an hour. The author doesn't reveal the location for obvious reasons.

Chinatown. The stretch of tunnel goes from near Cyrystie Street, under the Manhattan Bridge approach plaza, towards Confucius Tower.