Story-related.
Unlocked during Chapter 5.
After being escorted to the Power Station by Sheriff Breaker and Barry in the Chopper, you meet Cynthia Weaver. She asks you to go and cut the power to the Transformer Yard.
Go outside, fight the handful of Taken in the small courtyard on the right, head around to the right again where the bridge controls are to get to the Transformer power switch.
The solution to the bridge puzzle is to press Switch #3 (the right most Switch), followed by Switch #2 (the middle Switch).
This unlocks after solving the bridge puzzle and flipping the switch to shut off the power.
Below is a video guide solution to the bridge puzzle if you're having trouble:
Credit to YT User PerfectParadox for the video