This is an easy anime to get into as it has a little bit of everything from comedy to action to slice-of-life; all wrapped up with an interesting sci-fi setting with cool superpowers and likable characters. Everything is well executed in terms of pacing and development, but only if you watch without fillers, which contain a great amount of fan-service, and only serve to slow down the pacing to a halt.

Note: The dub is good for all iterations/seasons.

So Railgun is a side story to To Aru Majutsu no Index (A Certain Magical Index). Index is actually the main story while Railgun is the side story, but both series take place in the same universe and at the same time, interlacing with each other, just from different character perspectives. However, Railgun is the better story and since both are intertwined, I'm going to recommend watching them in the following sequence in order to avoid spoilers, which is partially based on the chronological timeline:

1. Railgun Season 1
2. Railgun Season 2 "S"
3. Index Season 1 (Concurrent with Railgun Season 2 "S")
4. Accelerator
5. Index Movie
6. Index Season 2 (Concurrent with Railgun Season 3 "T")
7. Railgun Season 3 "T"
8. Index Season 3

This is because Index spoils Railgun (which is the better story), while Railgun won't spoil Index. Furthermore, Index will continue the story after the events in Railgun.

And if you are like me and don't care too much about the slice-of-life & excessive fan-service, then I recommend you watch it without fillers: 

Filler Source:

• Railgun Fillers
• Index Fillers
• Accelerator Fillers

WATCHING GUIDE:

• Watch Episodes 1-2
• Skip Filler Episode 3
• Watch Episodes 4-12
  • (The events up to Episode 14 predate Index S1)
• Skip Filler Episodes 13-21 
  • Yes, the majority of Season 1 is just pointless slice-of-life & fan service but the story gets better in the next season. That is why watching without fillers creates a much better story with good pacing.
  • Even if you like fan service, I would recommend you come back to the filler episodes afterward, in order to maintain the pacing for the main story. The filler episodes are just side-stories, nothing important ever happens.
• Watch Filler Episodes 22-24
  • These are only canon to the anime but they are ok with good action scenes at the end and wrap up the storyline with the children.
  • You can ignore whatever you don't understand of the beginning of Episode 22 at the hospital and any unintroduced new characters, as they are just leftovers from previous filler arcs and not relevant to the story.
• Skip Filler Episode 25

• Watch Episodes 1-6 (25-30)
• Skip Episode 7 (31)
• Watch Episodes 8-16 (32-40)
• Skip Episodes 17-24 (41-48)

Watch.

The whole season is canon. Ironically, Index doesn't have any fillers, but the story is told much better in Railgun, LoL.

To be honest, this season is pretty average, but it's the one that explains how this world of magic works and introduces all the characters, so it's an essential watch to understand everything else in the Index Series where all the plotlines and spinoffs converge. The next Index Seasons are much better in both story & animation. Having said that, you can postpone this season until you are ready to watch Index 2 & 3. Just see below for the essential episodes to watch for Accelerator.

Is it necessary to watch Index if you already watched Railgun?

No, but it's fun to watch the story from the perspective of the main male protagonist and you get to see other parts of the story that are not shown in Railgun, the same events are slightly different, Furthermore, the story in Index advances the plot further than in Railgun.  However, if you don't like it, then drop it at any time. Personally, I think Railgun makes Index worth watching, but not the other way around, if you are only going to watch one of the two, then you should only watch Railgun.

• Index S1 Episodes 10-14 are the condensed version of Railgun S, you may skip them if you like. The same events happen slightly differently.
• Index S1 Episodes 19-20 move the story forward, beyond Railgun S, and are relevant for Accelerator.

Does Index have the same story as Railgun?

Not at all, Index focuses on Magic and they are a completely different story, divided into many small arcs, but at some point, the plotlines converge and the story is the same just for one Arc of Index S1 (Episodes 10-14), which is the focus of the full season of Railgun S, so the story is better developed in Railgun and with better animation & action, hence the recommendation to watch Railgun first in order to avoid spoilers.

Railgun focus is on science (fiction), more specifically espers and their powers; and has many references to real science like Synesthesia and the moral use/abuse of clone technology.

Watch.

This is a continuation of the events of both Index S1 & Railgun S (S2), from the perspective of Accelerator.

The First Episode is filler, but you would want to watch it as its the intro to this series.

Do you need to watch Index/Railgun first?

Yes for both, but you could start with Railgun S (S2) which is where the character of Accelerator is more relevant and introduced much better than in Index, then you could just watch Episodes 19-20 from Index S1, which are the relevant episodes that develop the story further than Railgun S.

Watch. (See below)

Chronologically, the movie happens between Index S2 Episodes 7 & 8 (31 & 32), so between The Remnant Arc (which will explain the injuries to one of the characters in this movie), and before The Daihasei Festival Arc. But since its just a side story, you could watch it anytime between Index S1 & S2 or at the end of Index S2.

Continuation of Index S1 and Railgun S (Season 2). And starting with The Daihasei Festival Arc (Index S2 Episode 8 (32), the events happen concurrently with Railgun T (S3).

You could easily watch the whole season in full (as there are no fillers) and then move to A Certain Scientific Railgun T to watch in full as well. 

Or you can Watch by Story Arcs to make it more comprehensible, which is what I recommend, just follow this guide:

• Watch Index II Episodes 1-7 (25-31)
• Watch A Certain Magical Index Movie: The Miracle of Endymion 
  • Index II Episode 7 (31) explains the injuries to one of the characters in this movie.
  • You could watch it at a later time if you prefer as it's a side story, but this is the ideal time to watch it chronologically, although it doesn't fit perfectly due to some inconsistencies.
• Watch Index II Episodes 8-13 (32-37) Daihasei Festival Arc
• Watch Railgun T Episodes 1-15 (49-63) Daihasei Festival Arc 
  • Episode 1 (49) happens 3 days before the Daihasei Festival.
  • This story is different from what you saw in Index II Daihasei Festival Arc, so you won't get bored. They just happen concurrently to each other.
• Watch Index II Episodes 14-24 (38-48) Includes: 
  • La Regina del Mare Adriatico Arc - Episodes 14-16 (38-40)
    • The prologue of the arc occurs on the last day of Daihasei Festival, September 25. The rest of the arc occurs in the span of a single day, September 27.
  • Academy City Invasion Arc - Episodes 17-23 (41-47) 
    • The entirety of the arc occurs in the span of a single day, September 30.
  • Skill-Out Uprising Arc - Episodes 23-24 (47-48)
• Watch Railgun T Episodes 16-25 (64-73) Dream Ranker Arc: 
  • Railgun T Episode 18 (66) - Bust Upper
    • Hamazura Shiage working for ITEM shows that this episode takes place after the Skill Out Uprising Arc (so after Index II Episode 48).
  • Railgun T Episode 20 (68) - Ha det bra
    • The events up to this episode happen before Index 3 Battle Royale Arc (Index 3 Episodes 4-6 (52-54)).
    • Then skips forward in the timeline to one week after Saten's last seen contact with Frenda, the night where Saten sees Uiharu instead of Frenda. Uiharu's injury puts this night after the Battle Royale Arc (so after Index 3 Episode 6 (54) where you get to see what happened to Frenda).
  • Railgun T Episodes 21-25 (69-73) - Dream Ranker Arc (Doppelganger) 
    • These events happen sometime between Index 3 Episodes 6-7 (54-55), so between the Battle Royale Arc and the Acqua of the Back Arc.

Watch. 

You can either watch this season in full or by Story Arcs, as indicated above.

Everything is canon. This first part of this season is concurrent with Index II starting from Episode 8 (32). Railgun T goes further with the story than Index 2 and the second part is concurrent with Index 3.

Watch.

• Watch the whole season in full (as there are no fillers).
• This season is much darker than anything seen before, so this change of tone might turn off some viewers. There is a lot of unceremonious killing, so be prepared for that. But for those who like action, this season is one of the best.
• Index is the place where all stories converge, but unfortunately because it's released earlier than any other stories, it just becomes a scrambled mess, with lots of characters that seem important, but you don't know who they are? what they are trying to do? how are the events relevant? etc. I recommend you watch everything else first, so you can understand and appreciate this better, but it's evident that future seasons of Railgun and/or Accelerator will probably clarify the events on this season better.
• The First 6 Episodes happen concurrently with Railgun T (see above in the guide for the Story Arcs to see how they intersect with Railgun T).

Optional.

It's average filler comedy & slice-of-life with fan-service.

Skip.

Just random comedy sketches.

Skip.

Unless you like fan-service, this is just a beach episode.

Skip.

More fan-service.

Skip.

Random comedy sketches.

Skip.

Unaired episodes included in the 1st and 5th BD/DVD volume releases of A Certain Scientific Railgun T.

I haven't seen these and I don't plan to, given the quality of other specials.

Skip.

Chibi comedy related to the Index Movie.

Skip.

I haven't seen these and I don't plan to, given the quality of other specials.

Skip.

Chibi comedy related to the Index Series.

Skip.

Chibi comedy related to the Index Series.

Skip.

Chibi comedy related to the Index Series.

Optional, but recommend skipping.

It is really just all characters mash together into a J-Pop music video, only fans of that genre may want to give it a try.

'Well it's a short animation to celebrate Kamachi Kazuma's work as a novelist (it features characters from his different series)

I think it was pretty enjoyable! (And I got to see Othnius *^*)' -SogiitaGunhaGUTS