I guess...
- Railgun I
- Index I
- Railgun II
- Index II
Railgun I is different, but I had a feeling It's in the same timeline as Index I or before that
also in Railgun II, there's some scene that can be seen in Index I too
and in Index II, Misaka sisters are still exists
but They're still a different canon, or more likely.... Railgun is a side-story of Index