According to the official website, an extra anime will be bundled with the limited edition of Toaru Majutsu no Index II Blu-ray and DVD volume 1. It will be released on January 26th, 2011.
A Side story novel of Toaru Kagaku no Railgun will also be bundled with all the volumes of index Blu-ray and DVDs. Misaka Mikoto is invited to Russia to demonstrate the ability of Level 5 at a shopping mall. But she feels a threatening atmosphere in the city. Mikoto meets a girl called Lessar and fights against the shopping mall with her.
Source: Index official website
Still not finished with Railgun.. *sigh* I hope somebody might sub this extra ep. 🙂