The expanded A Certain Magical Index universe came back with a vengeance in 2018, continued with A Certain Scientific Accelerator in 2019, and will be rounding out its huge comeback with a third season of A Certain Scientific Railgun next year. But given how the main series can run for over 20 episodes each season and the spin-offs can generally cap off at 12, it's been a question as to how long this newest season would run for. According to new listings on the anime's official website, A Certain Scientific Railgun Season 3 is officially slated for 25 episodes.
The official website lists the home video releases for the series as having 25 episodes spread out over eight volumes of Blu-ray and DVD releases. The first and fifth releases will include an anime bonus, but it's currently unclear as to what this means. The Twitter account for the series also revealed new cast additions:
・削板軍覇 役 河西健吾 さん
・警策看取 役 富田美憂 さん
・ヘソ出しカチューシャ（雲川芹亜）役 藤井ゆきよ さんDecember 25, 2019
These new additions include Kengo Kawanishi as Gunha Sogiita, Miyu Tomita as Mitori Kozaku, and Yukiyo Fujii as Seria Kumokawa. These three join the previously confirmed returning cast of Rina Satou as Mikoto Misaka, Satomi Arai as Kuroko Shirai, Aki Toyosaki as Kazari Uiharu, Kanae Ito as Ruiko Saten, and Azumi Asakura as Misaki Shokuho.
A Certain Scientific Railgun's third season, dubbed A Certain Scientific Railgun T, has yet to be officially confirmed for an English language release as of this writing. A Certain Scientific Railgun is set in the same world and around the same time as the main series, A Certain Magical Index. The spin-off focuses on Mikoto Misaka, a Level 5 electromaster esper who is at the top of Academy City but gets embroiled in all sorts of trouble after befriending Toma Kamijo and Index. With two seasons of its own under its belt, you can currently catch up with the anime before the premiere of its third season on FunimationNOW and Hulu. For those wanting to check out the main series where it all began, you can find its three seasons on FunimationNOW as well.