Debuting in 2004 as a light novel series by Kazuma Kamachi, A Certain Magical Index is among the most influential franchises of its type. While the original intention was to change the title of each volume to better reflect that arc's main character, this concept was ultimately dropped in favor of establishing Index as the principal title. However, the light novels spawned various side-stories, including A Certain Scientific Railgun SS, a manga spin-off series centering around Mikoto Misaka.
A Certain Magical Index and A Certain Scientific Railgun were adapted into anime by J.C. Staff, with the latter's third season set to debut in the Winter 2020 season. While both series are inherently connected, Index and Railgun have quite different strengths and weaknesses. Which one is the best? Well, let's find out!
Disclaimer: The focus is on the anime adaptations and not the light novels or manga.
10 Railgun: Mikoto Misaka Is An Incredible Protagonist
Known as the "Railgun," Mikoto Misaka is a Level 5 esper with the ability to generate and manipulate electricity. In Index, Mikoto is mainly reduced to a potential love interest for Touma, with the anime amplifying the character's tsundere traits to an annoying degree. Freed from the constraints of portraying a supporting character, as Railgun's protagonist, Mikoto blossoms into a truly likable and endearing character.
Compared to Touma's rather bland anime version, Mikoto feels far more realistic and three-dimensional. Depending on the situation, she can be arrogant, childish, self-destructive, shy, and kind-hearted. Biri Biri is electrifying.
9 Index: The Magical Component
As implied by their titles, A Certain Scientific Railgun and A Certain Magical Index focus on the scientific and magical sides of Kamachi's fascinating universe. For the uninitiated, Index takes place in a world were certain individuals known as Espers are born with powers, although these abilities vary greatly from person to person. Those born with such traits attend Academy City, a hub dedicated to scientific discovery and advancement.
Magicians exist on the opposite end of the spectrum, as their powers have no scientific basis and are typically dependent on mastering arts and mythologies. These two sides are constantly in conflict, even if most Espers are unaware of the existence of Magicians. Railgun takes place exclusively in Academy City and focuses solely on the scientific side of the equation, while Index provides a more complete picture of this universe's central power struggle by dealing mainly with Magic.
8 Railgun: Slower Pacing
Admittedly, this might be seen as a negative by some people, but Railgun does not rush through its arc and even allocates episodes to simply showing the four main characters living in Academy City. Railgun's first season, especially, almost qualifies as a slice of life anime, even though this aspect diminishes as the series progresses.
Index is infamous for rushing through its arcs, with the third season being the biggest culprit. In fact, the anime adaptation can be extremely difficult to follow as J.C. Staff seems unwilling to let the characters breath. Railgun does not suffer from this problem.
7 Index: So Much Accelerator
While Accelerator features prominently in Railgun's best arc, the Level 5 Esper is practically a secondary protagonist in Index's second and third seasons. The most powerful Esper in the world, Accelerator starts out as a villain who sought to become the first Level 6 Esper; however, once Touma introduces his fist to Accelerator's face, the latter begins to reexamine his life choices.
Accelerator's transformation into something resembling a hero is reasonably well-executed, especially through the Esper's relationship with Last Order, a clone of Mikoto Misaka. More Accelerator is always a good thing, and Index delivers that in spades.
6 Railgun: Anime Touma Works Better In A Supporting Role
Possessing the Imagine Breaker in his right hand that cancels out scientific and magical powers, Touma is a great protagonist in the light novels; unfortunately, the anime does a poor job of adapting Kamachi's hero. Without inner monologues to show that Touma's countless righteous speeches addressed to the villains are just distractions to allow the teenager to find an opening to deliver a punch to the face, Touma ends up coming across as a somewhat bland, shallow, and even overpowered protagonist.
As a secondary character on Railgun, Touma benefits from being less overexposed and manages to come across as something of an enigma, as the spin-off does not reveal anything more about the character than what Mikoto happens to know.
5 Index: No Filler
There is no filler in Index's three seasons, hardly surprising considering the original light novel series ended in 2010. Comparatively, Railgun's seasons tend to be evenly divided between canon and filler arcs; in fact, nearly half of the first two seasons qualify as filler.
Putting aside the boring "Silent Party Arc" in A Certain Scientific Railgun S, most of Railgun's filler episodes are pretty decent. Nevertheless, quite a large percentage of the anime can be skipped over without missing anything of significance.
4 Railgun: Index Librorum Prohibitorum
Despite what the title might suggest, Index is anything but a positive in Index's favor. Possessing the knowledge of over 100k grimoires, Index is a crucial character who ends up in Touma's care at the beginning of this grand adventure. Although her presence declines somewhat following the first half of Index's opening season, she remains a fixture throughout the anime's entire run.
Unfortunately, Index also happens to be the worst. Mooching off Touma and often resorting to violence whenever her caregiver displeases her, Index is a frustrating and unlikable stain on the franchise that has shown little to no growth. While other characters develop, she remains static, serving as an anchor that drags the rest of Index down into the gutter. Luckily, Index is practically a non-entity in Railgun.
3 Index: Shirai Kuroko's Reduced Role
Both anime adaptations exaggerate comedic traits to the detriment of the acting character. No other player suffers more from this issue than Shirai Kuroko, Mikoto's best friend who takes every opportunity to harass and invade the protagonist's personal space.
In the source material, Kuroko mostly does this to distract Mikoto whenever she is feeling down; in the anime, Kuroko does this endlessly and regardless of Onee-sama's emotional state. This is also not treated as a harmless joke between friends, as Mikoto clearly feels uncomfortable whenever Kuroko decides to cop a feel. As Kuroko is only a bit-part player in Index, she is allowed to shine without coming across as a deranged stalker.
2 Railgun: The Sisters Arc
With the exception of Index season 3 and the previously mentioned "Silent Party Arc," most of the anime franchise's arcs are perfectly watchable. That said, only one storyline stands out as a true masterpiece: "The Sisters Arc."
While Index rushes through this brilliant story during its opening season, Railgun dedicates 16 episodes to covering Mikoto's struggle to stop certain individuals in Academy City from using clones of herself as expendable pawns designed specifically to be killed by Accelerator in pursuit of that coveted Level 6 Shift. Railgun is worth watching simply for "The Sisters Arc," a storyline that ranks among the best the anime industry has to offer.
1 Index: It Is Still The Main Series
Regardless of quality, Index is still the main series. Railgun might handle certain arcs (and characters) better than Index, but the latter is more relevant when it comes to worldbuilding, plot development, and character follow-up. It is not unusual for someone to play a notable role during one of Railgun's arcs before disappearing without much of a follow-up; however, that seldom happens on Index.
Both anime should be watched to get a complete picture, but Railgun's overreliance on filler and limited scope means that it will always be a side-story to Index. In this case, the side-story just happens to be better.