[Jan-Poo]

Originally Posted by Clarste

Everything in Season 1 of Railgun happens before Index (well, not technically true, but close enough). Railgun season 2 happens during Index season 1. This is the same arc from another perspective.

Touma doesn't actually that big of a part in this arc. He just pops in at the very end to save the day. Mikoto had a lot of offscreen troubles she briefly refers to in the Index anime. Touma gets more scenes than usual though, I guess.

There's something that doesn't seem to fit.

The first season of Index starts with what seems to be Touma and Misaka's first encounter. That whole scene would hardly make sense if it happened after everything that we see from Railgun S1

Moreover Index appears in the EP19 of Railgun S1

Spoiler for Index S1 and/or Railgun S2:

It should logically follow that Railgun S1 at the very least happens after the first few episodes of Index S1 and before EP10 where the clone of Misaka appears.

Spoiler for Minor even in Index S1:

I'm not sure if I pieced everything together, but is there a mention of the satellite that was destroyed by Index at the early part of S1 in Railgun? If my reasoning is correct, it should be already destroyed by the start of Railgun S1.

Still this would mean that the whole Railgun S1 happens in the same span of time that in Index S1 is covered by merely two episodes...