Route 315 is the route connecting Route 312 and Blackfist City together. The Thunder Underroute is also present in the middle of the route, and serves to create an obstacle for the player to navigate through in order to access the remainder of Route 315 and Blackfist City.
With the events on Mount Press over and done with, the player now has full access to Route 315 through the entrance on Route 312 that is no longer blocked by Team Olympus/Asgard Grunts. Upon entering the route the player has three separate path options that they may take from the entrance to Route 312: A path to the east, a path to the south, and a path to the west.
The path which leads to the far east of the route takes the player around a boulder before leading them to a grassy area with a single rock in the center. Interacting with this rock reveals that this is the Moss Rock, which can be used to evolve an Eevee into a Leafeon when leveled in the surrounding area. The path then winds southwest for a bit before forcing the player to jump over a ledge which then positions them directly to the right of the entrance to the Thunder Underroute.
The western path leads the player south and then forces them to travel eastward until they eventually reach the entrance to the Thunder Underroute. On this path the player will encounter a few small ponds which can be used for finding wild Pokémon by Surfing but not fishing.
The southern path causes the player to jump over a ledge which then puts them onto the same path as they would have traveled on had they taken the western path instead. This is therefore a shortcut and it is also the only path where it is possible for the player to avoid contact with other trainers along the way.
Cool Trainer Jeanette
Pokémon Ranger Arnold
Black Belt Colreath