I was just wondering if any enemies in any of the metro tunnels ever respawn once you clean the place out. Do any enemies in any interior cells respawn?
I know they do outside.

No, the interior spaces don't respawn at all. And not all of the exterior enemies respawn either, it depends on where you are. In the Wasteland itself they almost always do, but once you get inside the DC area only some of them do.

Good to know in case I ever decide to start traveling on foot more. I always avoided walking through DC when I could because I didn't want to keep going down into the metro and fighting Ghouls. I hate ferals. Nice to know I only need to clear them out once.

If you get the ghoul mask from Roy (Tenpenney Tower quest) then ferals will not be a problem.

The funny thing to do is wear it where you know ghouls and raiders or ghouls and super mutants are and lure the raiders or mutants into the ghouls. Its really great if you can get a reaver to do your work for you.