User blog comment:Jethro Harper/Just from what I've seen so far/@comment-209.6.232.44-20160812112709

Railgun was licensed by Seven Seas who also have the license on the Accelerator Manga and they are either caught up or only one volume behind the orignal release. The Index novels and manga are licensed by Yen Press who seem to have a different approach. Yen Press relases the novels every 3 months and the manga every 4 months. Since Yen Press only release the novels at about the same pace they are orginally published 4 novels a year, the novels will never catch up. The manga's bundle that year's chapters and come out about once or twice a year so the translations will eventually catch up to the orignal langauge release.