The Brooklyn-Manhattan Transit Corporation, or BMT, ran all the below-ground subway and above-ground trolly lines in Brooklyn and parts of Manhattan up until 1940, when the City purchased the BMT along with the other large agencies to create the current MTA.
In this vintage map from 1930, the above-ground (surface) lines are mapped in amazing detail. It's amazing how much of an even spread they got across the borough, much better than the current subway layout.
via Brooklyn Rail
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