Reason: Former is a spinoff manga of the latter novel/manga/anime, some shared cast. Only two or three images tagged Railgun so far have any indication of being Railgun and not just Index. There seems to be a tendency to tag Misaka Mikoto and Kuroko pics with Railgun, since they get more focus there.
More Railgun-specific art/characters might follow later since an anime of it has been announced, though.
Updated by jxh2154