There are a total of 4 Destructible Objects located in the Port Adams Subway and they can be found in the following locations:
Breakable Object 01
As soon as you get off the lift upon entering the Subway, look to the right and upwards. Towards the top of the wall here you’ll spot a militia shield.
Breakable Object 02
Mounted on a circular doorway along the right hand wall just before arriving at the first ramp.
Breakable Object 03
After going over the first ramp, drive a short way through the tunnels until you reach the first obstacle blocking the path. At this point, turn around and look above the entrance to find the militia shield.
Breakable Object 04
After going through the subway station you’ll find yourself in a room with three wooden walls we can drive through. Go through the door directly in front and when you reach the large rock in the centre of the room, look up on the wall behind it for the final militia shield.