Board Thread:Questions and Answers/@comment-103.19.76.6-20150410101648/@comment-125.60.240.201-20150410105156

In the Toaru Kagaku no Railgun/A Certain Scientific Railgun manga only the later half of the Sisters Arc have parts adapted from the third Toaru Majutsu no Index light novel volume, and even then it is modified to accomodate Misaka Mikoto's POV not Kamijou Touma's.

For the most part it is an original story running parallel to the story of the Toaru Majutsu no Index novels, a side story. But as we have seen recently the manga demands the reader to know what has happened in the Toaru Majutsu no Index novels for context, for afterall, the story in Toaru Majutsu no Index affects that of Toaru Kagaku no Railgun.