... you do know that not all vent grates belong to the subway, right?
Those are Con Ed underground transformer vents.
Heres what to look for.
Most, if not all subway vents are sealed on the surface... because we can access them from beneath, in the tunnels if we had to. also, for the most part, we have no reason to go in there except to clean them out. They're for air circulation only.
Con Ed vents, on the other hand, have hatches built into them so workers can climb down inside, because there is hardware down there that needs maintaining.
Or else this can happen