Toaru Majutsu no Index, Zettai Karen Children: The Unlimited - Hyoubu Kyousuke Recommendations
One of the main things that makes them similar is a whole super power thing.
In both series we have a girl who can teleportate and a guy who can nullify any of those attacks.
In a way, both of these series reminds me of one and the other.
Both series features the theme of espers who inherits supernatural abilities in a city like setting. There seems to be drama, action, and some comedy presented in both series. Zettai Karen Children: The Unlimited - Hyoubu Kyousuke has a more serious feeling with Index maintains more of a lighthearted atmosphere. However, both series explores the idea of espers and their powers.
Both series also has a super power like theme.
TMnI is about magicians & espers, but ZKC is only about espers that often fight against each others
The white haired badass had lolita compl-... oops I mean a loli near them xD
And a boy who could nullified other ESPers powers
The stories not centered in one main character
Shounen full of action & supernatural elements
Oh, Index's stories is included in Railgun too :)
Well, in Railgun there's no magicians
Well doesn't matter which one you like.. But ZKC & Toaru series is so similiar in many ways
I highly recommended ZKC for Toaru fans and Toaru for ZKC fans
Just try 3-4 episodes, and you won't stop from watching one of
Both anime series involve Espers with unique abilities, and both main characters have the same ability which is the power to repel/negate the abilities of other Espers.