Getting to the Red Key Slot (10)

In the main station area, open the double doors to reveal a Commander and an Enforcer. Defeat them.

A Chaingun Cannon is on the table, and Chaingun Cannon Ammo is on the left bookshelf.

SECRET 4: Go behind the red curtain at the back of the room to find two Freezer Ammo and an Atomic Health.

To exit the building, go onto the roof. You can find Devastator Ammo and a Shotgun there. Be aware that some Sentry Drones (flying robotic enemies) will spawn on the roof, so using a Jetpack (a power-up allowing flight) might be a safer approach to collect these items.

Return to the waterway and emerge at the Red Key door you've seen previously.

Warning: Prepare for a difficult fight as three Enforcers and two Battlelords (powerful armored enemies) will be waiting. Use the water to your advantage by going in and out as needed.

After defeating the enemies, a switch on the wall opens a compartment containing an RPG (Rocket Propelled Grenade launcher).

There is a crack in the wall that can be destroyed to proceed.

Inside the crack, you will find a couple of Large Medikits on a ledge and the Exit Symbol shortly after.