[RBomber]

Okay, just bring things up.
Thanks to selkirk, he had translated the prologue chapter of the light novel.

Okay, now anyone was wondered why Mikoto got very pissed off at Touma, right?
Well, it figures, that in the light novel, Touma

Spoiler:

actually saved the thugs from being deep-fryed by Miss Railgun, by purposely taunting and mocking them (so they change their target to Touma instead). Add that Touma had met her before (and not on friendly term(?)), it is quite understandable that Miss Railgun is genuinly pissed off.