Shortly after completing the first Orphanage quest, part of the floor gets a crack in it, making it require strength to past.
After getting strength and the strength badge, a new area of the Railnet opens (With no wild Pokemon), allowing you to visit the inner part of Obsidia Ward and Yureyo Headquarters. After getting the key you unlock another pert of the railnet: the puzzle room. The Puzzle room has a bunch or tracks and levers that you need to connect (without any obstructions) in order to open two doors, the one on the left leads to the Cave section of the Railnet, and the one on the Left leads to Chrysolia Pass.
Railnet Cave Section
The Cave room is dark, so TM Flash is very useful.
Help! I've Fallen and I Can't Get Up!
Just as you enter the cave room, just above you there is a smash-able rock. Behind it, there is a man who fell from into the street from Obsidia Ward and couldn't get out. Upon talking to him he leaves to go to Obsidia Pokecenter. Go there and talk to him, and he will give you a department store sticker.
At the bottom of the cave room, to the far right, there is a dark section of wall between two rocks. Use rock smash on it and TM flash will be inside.
At the bottom of the cave room, there is a metal beam that looks as though it can be headbutted. When you do (It may take several tries), an Aron will fall down.
Cave Technical Readout
At the bottom of the cave room, to the far Left, there is a dark section of wall between two rocks. Use rock smash on it and the Cave technical Readout will be inside.