There are a bunch of partially completed stations (or expansion provisions for future stations) which were intended for the "IND Second System" -- a massive set

There are a bunch of partially completed stations (or expansion provisions for future stations) which were intended for the "IND Second System" -- a massive set of projects that got killed off by the Great Depression. (The beginning of one of those lines actually got built and put into service, in the form of a one-station spur line branching off Hoyt-Schermerhorn in Brooklyn, and going to Court Street, where the line now stops. It was originally intended to continue into Manhattan and become the Second Avenue Line. Instead, it was operated for a time as a shuttle, then discontinued. Court St. Station is now the main site of the Transit Museum, which may reopen one of these days. There's now a rump Second Avenue Line recently put into service on the Upper East Side -- three stops of it -- but construction of the rest isn't fully funded, it's not planned to reach lower Manhattan for a few more decades, and there are no active plans to continue it into Brooklyn.)