Where is the first track of Toaru Kagaku no Railgun S OST 2 first played? The OST in question can be found here. I'm looking for where it was played in the anime.

The title says "Saijaku VS Saikyou (最弱 VS 最強)," which translates to "Weakest vs Strongest," but I don't seem to hear it in Touma vs Accelerator's battle.
this song was featured on the Railgun S OST but it first appeared in "a certain magical index", episode 14 which was actually titled the same as the song, "strongest vs weakest", viewable here https://www.youtube.com/watch?v=7NV2fR2cdRQ
but since your post does request when it appears in relevance to the Railgun series that would be episode 4 when Touma is fighting Misaka under the bridge at night. here's a clip of it directly from funimation's official channel https://www.youtube.com/watch?v=-lEtG-CJ9IY 
in the Railgun S series it was swapped for the soundtrack from OST volume 2 #16 "Ketsui to kakugo" in the Touma vs Accelerator fight which took place in episode 15.