This article touches on but doesn't explore in great detail the history of elevated lines in NYC. Some of their traces have been incorporated into the current system, most notably as the 7 train in Queens and the J/M/Z (including the Ridgewood/Middle Village spur), but many were torn down.

Some lines in Brooklyn that people aren't even aware of (beyond the 2nd and 3rd Ave lines):

* The Myrtle Ave elevated, running from Downtown Brooklyn down Myrtle and connecting with the M's Ridgewood spur. Torn down in the 1960s.

* The Lexington Ave elevated, running from the Brooklyn Bridge waterfront across Downtown Brooklyn and Bed Stuy, connecting to the current J/Z line near Broadway Junction. Torn down in the 1950s.

* The Fulton St elevated, since replaced by the A train, that ran over Fulton St all the way into Downtown Brooklyn. Torn down in the 1940s.

You can see these elevated lines crossing Northern Brooklyn in this map from 1949: https://www.nycsubway.org/perl/caption.pl?/img/maps/system_1...

I personally would have found these lines quite useful, but what took their place was a collection of bus routes, running along old streetcar routes (read: weird routes that stop suddenly at borough and neighborhood boundaries). And NYC buses cannot be trusted...