Index-Railgun is looking to surpass 100 episodes with its four seasons and 1 movie so far, but however you look at it, this is way too many. For people who want to understand why this is the 14th best anime of all time, but don't have the time or patience to wade through all the nonsense content between the genuinely good parts, I present to you the condensed Index-Railgun guide. These episodes should be 90% of the nutritional value of the series with 10% of the calories:
Our first goal is to make Index-Railgun chronological, which is actually quite difficult given that the release schedule of the series has not been. For that reason, first, watch:
Railgun Season 1 episodes 1-12. Railgun is an awesome series when it follows the manga, so have no fear, all of these episodes serve a purpose and are well made.
Next, switch to Index season 1 episodes 1-6. This is necessary for important details concerning Index, Touma, and the Tree Diagram, and falls chronologically right before Railgun Sisters.
Next, watch Railgun Season 2 episodes 2-6 and the beginning of episode 7 until it stops featuring Misaka and switches to Saten (the 5 minute 40 second mark to be exact). Then move on ahead to Railgun Season 2 episodes 8-17. Congratulations, you've finished the Sisters arc, the main value of Index-Railgun.
Now, watch Index Season 1 episodes 15-20. This includes Touma's touching reunion with his parents, a date with Misaka, and Accelerator's redemption plus more Sisters goodness.
Next up, watch Index Season 2 episodes 6-9 because it's mainly about Misaka, Accelerator and Kuroko and has cute Touma-Misaka flirting. Do not worry about the stupid bad guy Touma is currently fighting, suffice to say he wins and we can move on.
Next up, watch Index Season 2 episodes 17-24, because it's about Touma, Misaka and Accelerator working together in a dream team to save the day. It also has a date between Touma and Misaka, and a date between Accelerator and Last Order, so it's basically perfect.
Strangely enough, even with all the episodes I skip, I don't think there will be much missing or confusing. The important plotlines and characters are addressed, and the unimportant ones are unimportant.
I don't believe any of the remainder of Railgun Sisters will be useful, so you can just ignore it for now. The movie is also probably safe to skip since it isn't integral to the timeline. After this, it's just time to wait for Index III or Railgun 3, whichever one comes out next.
With this condensed version, Index and Railgun combined are only 51 episodes long, half of its unabridged size. I'd even venture to say that watching these 51 episodes would make Index-Railgun a superior viewing experience to watching all 100, because you cut out so many annoying, stupid, repetitive, unimaginative scenes that are net negatives to the series. If you just watched these 51 episodes, you'd be left thinking the author is a genius. (Which he is, when he isn't being stupid.) Also, by watching in the correct chronological order and without repeat material, the series will make a lot more sense than how it was presented originally.
Index-Railgun is a truly great work of art, so I encourage everyone to check it out in this abridged form, in this chronological order. If you enjoyed it and want to see more, you can always fill in the gaps afterwards.