So I talked about A Certain Magical Index series and its spin-offs in the previous article. All of Toaru/Raildex universe arcs through various works (anime/manga/novels) share the same timeline and unless stated otherwise, any event in series is canon to all of them.
Since there are many of works that share the same universe it's hard to decide where to start. While most of Index/Railgun stories are self-contained, characters and plot-points can be introduced in one series then appear without introduction in the other.
So, which order should I watch the series?
First of all: It's essential to watch/read at least the first arc of Index. (Ep 1-6 of the anime. Volume 1 of the novels. Vol1-2 of the Manga.) The first arc of Index contains most of the needed world building and establishes Kamijou Touma's character. If you watched Railgun first (especially when you reach Railgun S,) Touma's interactions might feel random as Railgun is strictly a Science-side story while most of Touma's conflicts are with the magic side.
The short/easy version:
Index I -> Railgun -> Railgun S -> Index II -> Index III > Accelerator -> Railgun T
I would advice people who have time to watch the anime series in the order they were released. (The order above is similar to, but not exactly the release order.) The animation gets better with every new season (Railgun T is better animated than Index III which is (arguably) better than Railgun S and so on.)
The Chronological Order Version:
Another way to watch the series is by watching in Chronological Order. Watching ichronologically order isn't always better. The writer assume that you have watched/read everything that was released up to that point while using a character/plot-point earlier in the timeline, using the prior-knowledge of the fans to spice the story up!
For example: Sisters Arc in Railgun is written assuming the audience have watched the same arc in Index first. It glosses over Kamijou's part of the story while giving a huge development to Misaka before and after the main events of the arc. Using their knowledge against them to spur emotions.)
Here's the Chronological Order of the major events, I won't mention some arcs because they're not too relevant to my list. Don't skip them though. Every arc progresses its respective series plot in a way.
- Level Upper Arc (Railgun ep01-14.)-
- Index Arc. (Index ep01-06) [Happens at the same time as the latter Level Upper Arc events.]
- Sisters Arc (Railgun.) (Railgun S ep01-16. Same arc below: Misaka's perspective.)
- Sisters Arc (Index.) (Index ep10-14. Same arc above: Kamijou's perspective.)
- Three Stories Arc. (Index ep18-20 + Index II ep01.)
- Kazakiri Hyouka Arc. (Index 21-24.)
- Orsola Aquinas Rescue Arc. (Index II ep02-05.)
- Tree Diagram Remnant Arc. (Index II ep06-07.) [Explains Kuroko's Injuries in Railgun T.]
- Daihasei Festival Arc (Index) (Index II ep08-13)
- Daihasei Festival Arc (Railgun) (Railgun T 01-16) [Same time but no shared events with Index.]
- Dream Ranker Arc (Railgun T's Second Half.) [Spans several days. Multiple Index arcs.]
- Academy City Invasion Arc (Index II ep17-23) [This arc events results in the whole Index III.]
- Battle Royal Arc (Index III ep04-06) [Some Railgun characters were first introduced in the novel version of this arc in Index.]
I didn't include Accelerator's anime's Necromancer arc as I don't know where it fits in the timeline yet. Anyway, my recommendation for the chronological watching is as follows:
Index (01-06) -> Railgun (first season) -> Index (07-09) -> Railgun S (01-10)
Index (10-14) -> Railgun S (11-24) -> Index (15-24) -> Index II (01-13) -> Railgun T (01-16)
Index II (14-24) -> Index III (01-06) -> Railgun T (17-25) -> Index III (07-26)
Now that I researched/wrote the chronological order of the arcs above, I'm thinking of writing a full-fledged article about the timeline of the series... But I'll leave the decision about that to the future...