Separate 114-page volume bundled with the special edition of the Toaru Kagaku no Railgun manga volume 5. It contains special illustrations, comics and 2 mini-novels. A mini-novel by the author of the Baccano! and Durarara!! series, Narita Ryogho.
Misaka Mikoto finds herself fighting against the Rank 7 Level 5 of Academy City, Sogiita Gunha. However, a young girl turns out to be the culprit behind their unprepared display of abilities: a primary school student named Kihara Nayuta.

(Source: Toaru Wikia)