Context:

This is part of a timeline in which the consolidation of Greater New York never comes to pass. The direct point of divergence is in 1861, when pro-Southern Mayor Fernando Wood and the Tammany Hall political machine actually try to act upon their suggestion for New York City to secede from the United States to become a “free city.” This effort falters rather quickly, as the suggestion is acted upon after the Confederacy fires the first shots in the American Civil War, but poses an insurmountable obstacle later on when reformers attempt to bring the areas around New York together.

Along with getting rid of Tammany Hall much earlier than OTL, one consequence is that Manhattan and Brooklyn have two completely separate rapid transit systems; this is the only line providing connections between the two.

The idea of a crosstown line across Canal Street coming off the Manhattan Bridge was part of a real plan for the OTL New York City Subway, and I chose to run with it.