Originally Posted by Fred314X

There are more planned (and unbuilt lines) than the ones you've listed. The reason for the middle track at Bedford-Nostrand on the G line was to accommodate a new line that would have crossed Brooklyn west-to-east, then come aboveground and connected to the Myrtle Avenue (M) line at Broadway.

If you take the F train to East Broadway, at one end of the station you can see ceiling beams set at an angle. That's part of the tunnel that would've taken trains to the South 4 Street station in Williamsburg, from which five or six new lines would have branched out across Brooklyn.

And at Roosevelt Avenue in Queens, there actually is a lower level to the station, built for what was known as the Winfield Spur. Had it been constructed, there could've been a direct subway line to JFK.

The reason why the D line makes a turn to the east for the run between Bedford Park Boulevard and 205 Street is that the original line was proposed to run all the way across The Bronx. But after the city acquired the former NY, Westchester & Boston railroad trackage (it became the IRT Dyre Avenue line), Mayor LaGuardia decided that there was no need to have two lines serving the northeast corner of the borough, and so the D line was scaled back and never built beyond 205 Street.

And one more proposal you might be surprised to know about: when the Archer Avenue extension opened, Parsons/Archer was conceived as only a temporary terminal. There was a plan to take the Laurelton branch of LIRR off their hands, and provide 24 hour service on the J line between a new terminal at Rochdale Village and Manhattan. As you can see from looking at the map, it never came to pass.

I could go on for a while, but I think I'm getting tired of typing just at present. So...

To be continued (just ask!)