Railgun is a spinoff of Index. The stories have overlap, but can be watched separately. Some of the overall story's context is clearer if you watch Index first, but there's nothing wrong with watching Railgun first.
For the Index/Railgun purist (fan) the suggested viewing order is Index 1, Railgun 1, Index 2, Railgun 2.
Sorry, if ask but can you please look at my requests and tell if you have it?
https://www.anime-sharing.com/threads/request-211210-rj361498-tentacle-works-テンタクルワークス-賢者ちゃんとhな触手.1392118/
https://www.anime-sharing.com/threads/request-231104-rj01109498-takotubo-タコツボ-つくよヴィレッジ.1397470/
https://www.anime-sharing.com/threads/request-misotake-みそテイク-multiple-games.1404036/
The Mexashare links for parts 2, 3, 5, 7, 8, 10, 11, and 12 are dead.
The Rosafile link seems to be online, but for I can't download it.
Additionally, I can't download the file from KatFile or Rapidgator either.