The Bridge Key will be used to the Steam Terminal on the right Bridge along with the Steam Battery so you can across to the Production Zone.
How to get Bridge Key
Find the large painting in the Hub and Click on the dial at the bottom of the picture. This is a combination lock. The combination is the number from the mayor’s office: 0516. Starting with the innermost ring, click on each triangle to get the wheel to spin. Then click on it again when it’s pointing to the right number. The rings get faster so this can be tricky. When you get the combination right, a secret door will open. Go inside and you’ll be in Captain Ziggs’ room. Go to the left and pick up the bridge key (on the floor).