The last two episodes are a bit of a crossover in my opinion. It clearly leads into A Certain Magical Index's part in the Sister's Arc as Touma Kamijou learns of the clones' existence and the Level-6 experiment. This all happened in A Certain Magical Index, but here things are experienced from Mikoto's POV. The scenes are longer and they set up the events quite nicely I believe. There are a few things that are explained that I didn't quite get in Index. A minor hiccup is that not all the dialogue is not 100% the same as they were in Index, but this can be forgiven. One example is Misaka 10032 having to name the black cat. In Index, she and Touma were walking while this happened, while here they are standing still. Noticeable, but not life-threatening.
I praise Brittney Karbowski who plays Mikoto for giving arguably one of the best/emotionally captivating performances I have ever heard. She excelled as Mikoto in season 1, but season 2 just...it really showcased just how fantastic and beautiful a voice she has. Human highlight reel! She takes the dark journey and just makes the audience feel for Mikoto. Austin Tindle who plays Accelerator feels a little different in Railgun S. Different in a good way. Accelerator was awesome in Index, but here we get to see him right at the beginning. I've heard people complain that Tindle isn't scary as Accelerator. I watch Railgun S and I wonder to myself: Maybe I'm watching something different. This guy is freakishly creepy. Not only does he sound so aloof and uninterested in everything, but then Tindle flips the coin and this psychotic juggernaut steps out.
The animation is also miles better than Index and Railgun. Not that they were bad, but it all feels so much smoother and vibrant in Railgun S, which is ironic given how dark the overall story is. Good use of CGI. It's obvious in places, but really kinda blends into everything. Again, I've heard people complain, but I like it. Nice balance.I just don't get the different between good and great animation if people say Railgun S is average. I really don't and I don't mind staying ignorant of that difference.
One gripe I had was the pacing. It felt a lot like Fairy Tail. This is both good and bad as normally, Fairy Tail starts off slow, but usually at episode 4-5, shit hits the fan and all hell breaks loose. From there, things just get faster and more exciting and more faster and more exciting until - stop! You've reached the end. It's a technique that I rather enjoy for the most part, but there are always that first few slow episodes. Railgun S is a bit like that, but this was also true for the first season, so if you minded it then, you're probably going to here. Same is true for the reverse though.
Watch clip: Misaka confronts Accelerator