If you’re going to save Ada or Sherry you’re going to need the T-bar tool to open pathways in the Sewers area. It’s going to be a requirement to open the doors necessary to collect the chess piece plugs that are needed for the puzzle to open the doors.
The T-bar tool will be found outside of the Monitor Room near the Treatment Pool. Coming from the Monitor Room you will need to lower a bridge to get to this area. Once you have lowered the bridge and crossed you will look for the T-bar on a desk with a radio and a computer on it. The T-bar itself is red in color and will be lying on top of a cloth.
Once you have the t-bar there are numerous doors and shortcuts that can be opened in the Sewer area. The middle and lower levels have doors locked by t-bar and the underground facility’s upper area also has an area that can be accessed with the t-bar that will allow you to freely move back into the police station
You can find the T-Bar using the map below at the location in the image above and it will be a mandatory item that you need to progress the story.