Volume 4 of Railgun is where the first non-animated storyline kicks off.
Someone has been leaving cash cards all over the 7th school district, causing people to comb through alleyways and back roads to find more. Mikoto learns that the cards were left by Nunotaba, a 17 year old prodigy researcher who left the cards around to make people more aware of areas that are normally ignored...where unethical human experiments are taking place.
When Mikoto gave Academy City a sample of her DNA several years ago for "medical research", it was used to create clones for a project to create the first level 6 esper. The current highest-ranked level 5 needs to fight and kill 20,000 "Railgun" clones (that many because they all turn out to have less power than Mikoto- which shouldn't have surprised anyone, since Mikoto wasn't originally a level 5) in order to become the first level 6. (Although- why use clones of Mikoto instead of the second-ranked level 5 esper? Because she's the protagonist, duh.
Mikoto starts destroying the facilities involved in the level 6 project and encounters three body guards (including the fourth-ranked level 5) who were hired to protect a facility she has broken into.
Despite the light hand-waving (and one use of the word "genes" where "genotype" would have been a better fit :P), I really enjoyed these two volumes. The characters are still fun- aside from those who we are meant to hate-, the action is still exciting, the sprinkles of humor provide some needed levity (see: Kuroko's reaction to seeing Mikoto in a mellow, cheerful mood; Mikoto's interactions with a sharp-tongued clone), and the sad moments are suitably affecting (see: said clone's fate).
The content is darker than in earlier volumes. Where Kiyama had a good reason for doing what she did and a plan to undo the collateral damage, the villains in this arc are doing what they do because they can, ethics be damned. The people running Academy City who are involved in the experiment view their victims as guinea pigs. (Like Kiyama's students earlier.) The highest-ranked level 5 believes that serial murder is justified so long as it increases his power. (Granted, he doesn't consider it murder.) And the three body guards don't care who or why they fight as long as they are getting paid. There are also two surprisingly macabre scenes. (The damage isn't limited to bloodless electric shocks and teleport pin-downs this time.) It'll be interesting to see where the story will go next. (Die, highest-ranked level 5!!! > < Since I never finished Index, I don't know how this story arc will end. Please don't share what happens. Thanks! ^^)
Kuroko is still the yuri in this series. She mostly appears in volume 4 (volume 5 is Mikoto destroying the facilities), and in the more yuri-heavy 4-koma bonus comics at the end of volume 5. (My favorite being the one where Uiharu discovers Kuroko's Google search history. She is, indeed, one of us. :) )
Looking forward to the next volume!
Unrelated, but I finally saw Imagine Me & You this past week, and was struck by how similar it is to Moonlight Flowers.