[stillmissing]

Spoiler for timeline:

The bridge fight on the 19th in the 7th chapter of Railgun is the opening scene to Index. Touma meets Index the following day, the first official day of summer break. As the next chapters get translated, you'll see Mikoto's final encounter with Touma before his memory was wiped, and the story will continue past that point as Mikoto, Kuroko, and Kazari work to solve the level upper case while Touma is busy with the beginning of the Index storyline.

After that storyline is resolved the manga has gone into a special flashback arc of when Kuroko and Kazari were classmates in the 6th grade.