>>1685147>>1685150I remember posting these same pages a thread or 2 ago. There's not really any ENF aside from the lost top and the teleported swimsuit, although the rest of the chapter has assorted beach-related ecchi.
It's chapter 41.5 "Biri Biri Days" (easy to miss, since it's essentially a side-chapter/omake).
http://mangafox.me/manga/to_aru_kagaku_no_railgun/v07/c041.5/1.html 