You can find it at the start of the chapter. Turn the wheel to open the train gate. The secret area is at the top. Climb the structure once the gate opens.
Secret Area #2
You can find this secret area below the second bird portal. The only way to reach here is to climb up first from the bird portal and, then drop down on the right side.
This will take you to a lower level and you can reach the secret area after crushing wooden boxes and rolling.
Secret Area #3
After you have open the third train gate, continue the journey to reach the gate with the board ‘BITTER TEAR X-TRAKTION’.
Inside the building, go to the left side and climb up to reach the secret area after blasting the platform.
You can find the last secret area on the way to the open fourth train gate.
From the gate of the previous location, continue your journey toward the right. At the end use the money bars below the platform to reach the secret area.