Secret Level of Unit 1 - The Lost Station

The objective here is to find a powerful weapon.

U1L4W1

Exit the elevator and proceed straight own the corridor, picking up ammo, armor shards, and jump up on the crate in the corner to snag the Shotgun. Continue down, taking out any guards in your way and enter the first station through the door on your right.

U1L4W2 (strategy and secrets here)

Once in the station, curve around to the right immediately and dispose of the Enforcers. Grab the goodies and hop onto the highest crate and jump across to the dark side to grab the hidden silencer. Come back around to the station and jump into the water and turn left to enter a secret passage. Cruise up the stairs and collect the armor shards, Rebreather, health, and ammo. Hit the button on the wall to exit back to the entrance of the first station - but watch for the falling Parasite from the ceiling.

U1L4W3 (secret)

Go straight into the station again but this time step onto the moving car and let it take you down the shaft just a tad (there are two cars but only one has a health pack). Step onto the front of the car and jump into that secret compartment with health and ammo.  Jump into the water and kill all Barracuda sharks in your path - remember to come up for air if necessary. There will be a silver platform to your left o step on it to take you up to yet another secret area. Expect a Parasite, guard, and Enforcer as well. Walk to the end of this room and submerge back into the water. Turn to the right to collect any health you may need, then step onto the platform to your right. This will elevate to another hidden room and a window overlooking Station #2. 
You can go straight underwater and press the green button but this just takes you under Station 2 - we’ll get there another way.

U1L4W4 (strategy)

Shoot the enemies across the way through the other window and the Enforcers down below. Drop down and enter the doorway. Follow up the long corridor and enter the door at the top left of the stairs. 

U1L4W5 (secret)

Once inside, blow away the Enforcers on your upper left and enter underneath there to collect the armor shards and health - but stay in the corner until the laser blasters stop shooting from the wall (this is a trap). Jump across to the clear again but peak around to find the shootable glowing button above the blasters. Once activated, this opens a secret compartment to the left with knee-deep water and a destroyed cave. There are armor shards and Adrenaline down here. Take the platform back up - the blaster trap is now turned off. 

U1L4W6 

Exit this room and turn right to enter another room with plenty of guards. There is some ammo and armor shards lying around but proceed up the steps to the large room with the tower erected in the middle (enter from either side). Watch out for guards, Enforcers, and Flyers hovering above you. Walk down (or jump, depending which door you entered) and collect the power-ups on dry land and in the shallow water. Step on the platform to take you to the top of the tower and press the computer screen to lower the bridge, Walk across and collect your prize - a Super Shotgun. Your primary objective is now complete. 

U1L4W7 (secret)

When exiting, jump into the water to find the secret panel on the side of the tower. Shoot it to open and ascend into the dry area in the middle of the tower to collect some important power-ups. 

U1L4W8 (secret)

Leave this room by walking back up the stairs and exit the way you came - just be prepared for plenty more Strogg. When leaving the second set of stairs down, you’ll notice a yellow glowing button on the wall ahead of you, so shoot at it to raise the walkway. This allows you to grab the red key, and drop down into Station #2. Enter the car and let it take you back around to Station #1 to exit the level where you came in - the elevator back to the surface of level 3. 

U1L4W9 (Easter Egg)

Bonus: if you take the tram around 13 times, the train logo on the wall will be a picture of a yellow rubber ducky!