Watching order for "Toaru Majutsu no Index" and related anime I am slowly coming to the end of watching "Toaru Majutsu no Index" anime. There are a few anime series related to that world:


Toaru Majutsu no Index
Toaru Majutsu no Index II
Toaru Kagaku no Railgun
Toaru Kagaku no Railgun S


and some movies. But I am not sure in what order I should watch them. Someone told me that "Railgun" came after "Majutsu no Index" - should I watch "Railgun" first then, or the second season of "Index"? Are there some things that I can miss if I watch them in the wrong order, like not knowing characters that were already introduced? 

And if it is Railgun I should watch second, should I watch "Toaru Majutsu no Index II" afterwards, or "Toaru Kagaku no Railgun S"?
Or is it that it really doesn't matter and I can watch both of the "Toaru Majutsu no Index" series before the two "Toaru Kagaku no Railgun" series?
Having just watched them out of order (accidentally), I highly recommend watching them by order of release date, including the movie.
The original and spin-off series were produced by the same company. (The same team as well I believe) and never aired at the same time.
This makes the recommended order:

Index season 1
Railgun season 1
Index season 2
Index movie
Railgun season 2 (S)
Index season 3
Accelerator season 1
Railgun season 3

Enjoy!