Board Thread:Questions and Answers/@comment-35367719-20181219211413/@comment-37071781-20181219224205

You can start reading the Railgun manga anytime. Despite it being a spin-off of "A Certain Magical Index", many of its Arcs are exclusive to it (with one exception).

To put it simply:

The Arc between Vol.1 - 3 is manga-original

The Arc between Vol. 4 - 6 (and the first part of 7) is a development of an arc you have already read in the LN (but this time, it's from Railgun's perspective)

The Arc between Vol. 8 - 10 ( and the second part of 7) takes place during an arc in the LN; however, Misaka is doing her own thing - it is this particular plot that is exclusive to the Railgun manga.

The Arcs between Vol. 11 - 14 are exclusive to the Railgun manga.