The To Aru Kagaku no Railgun side story is based on Kamachi's original story set in Academy City, a city where about 80% of its 230,000 citizens are students. The story deals with the adventures of Misaka Mikoto, one of the Level 5 psychics in the To Aru Majutsu no Index light novels
Not a drop of it's original flavor is lost in this sidetracked series.
If you really want to go back to this universe and is really bored with waiting for the next season this a wonderful way to use up your time. Really ain't sorry for watching this sidetracked story.
This story is centered around Misaka and her band of friends getting and solving trouble. The story takes place before Toma even met Index, and after the middle of the series Index appears for a short moment. Toma appears from time to time to help a little but nothing big.
It did make some minor changes to the original story, but i'm sure you can ignore them and move on.
Only bad thing is it starts off a slow, but it get's were it wants to go in the end.
All in all I really enjoyed watching this series and can't wait for the next season of the main series.