So as well all know, Philadelphia has a decent transit system with one major gap, the Northeast of Philadelphia is completely unserved by either Trolleys or Subways. This is not intentional as a subway was planned for the Northeast, this subway is called the Roosevelt Boulevard Subway and would have been a branch line on the Broad Street Line, it would be expected to have 100k daily riders and is incredibly easy to build as Roosevelt Boulevard has a very wide median in place because it planned to always have the subway

Here is vaguely what it could look like on a map

And yes, it is incredibly long and would add a new station to the Market-Frankford Line as well