I wish I could have made it to the presentation. Was your question answered?
I’m pretty sure that “bridge” is the elevated train yards, aka Hedley Hall. Perhaps during construction, which this pic, from nycsubway.org, says was 1910.
The smokestack visible behind it in the OP, (still there today) makes me pretty sure that is what we are seeing, as do the angled tracks leading there from Broadway, visible coming out from behind PS7 in the OP.