it was already explained earlier in the topic.
To Aru magitsu no Index is the firs season of the story, Railgun is somewhere in the middle of it, and ends before the Misaka Sisters show up, you can pretty much measure the bits whenever he shows up in Railgun it means something happened in Index where they met, we are also assuming when he showed up on Misaka's performance he already had lost his memory by then.
Railgun season 2 will probably cover some more of Misaka Sisters part, but that will probably come out after Index season 2, or it could be something completely different.
Railgun season was done because ppl liked Misaka and wanted to know more about her, since in Index story revolves around Index and him, Misaka doesn't get much work in it.