Something I’ve been hammering on recently: NYC could build new light rail lines relatively rapidly and cheaply, especially in Brooklyn and Queens: much of those boroughs was covered by streetcar lines until the 1950s, and all of the streets that those lines ran on are still graded for that purpose. In some places they didn’t even bother to pull up the old rails, and just paved over them instead[1]. The roads themselves are owned by the city too, sidestepping half of the bureaucratic process that comes with the state-run MTA.
[1]: https://www.nycsubway.org/wiki/Tracks_on_Broadway