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.
Hi Ryzen, sorry to bother you but I was wondering, in one of you last upload RJ01032786 I noticed that only one track is present, where in DLsite there are listed 5 tracks. Is your upload correct to have just one track? Thanks in advance, and thanks for you work!