Depends on the show and network. ABC has a bunch that are captioned, NBC has some of their own and so does CBS. But not ALL shows/networks are CC'ed. Basically you have to look at specific shows to find out.
You should check out Hulu.com they have a bunch of different things that are CC'ed. Netflix now has a subtitle list of online instant play at netflix.com/subtitles (need a subscription tho).
BTW, Comcast and Xinifity are providers, not a network so it doesn't really have anything to do with captioning.