You'll come to a large chamber with a Gargantua fighting some soldiers. Don't stop to watch, because the beast will come for you next. Head quickly into the passage just ahead and to the right. You need to make it to the power generator. Unfortunately, the passage is blocked, and the place is teeming with Alien Slaves. Fight your way to the entrance to the track control room. The walkway will collapse beneath you. This is OK; you don't need to enter the control room yet. Turn around, and follow the hallway around.

Fight your way through the Headcrabs and Alien Slaves, following the signs to the power generator. You'll pass a health dispenser near the blocked stairway. Keep moving down the hallway until you come to a valve. Turn the valve to open the door to the hazard waste chamber.

Climb up the ladders to the top room. There are some soldiers and a turret waiting for you. Search the crates in the room, then head down the hallway. You'll come to a heavily fortified area. A soldier waits behind a barricade, but there are some explosive crates behind him. Shoot the crates. If you're in need of health, there's a small alcove just below the entrance ramp with a health dispenser and some Grenades. Head up the ramp behind the barricade. To the right, there's a number of soldiers. To the left there are a few soldiers, as well as a number of crates filled with supplies. Clear out the area to the left first, then head back down the hallway to the right.

A herd of Headcrabs will be dropping in from the ceiling. Let the soldiers and the Headcrabs fight each other, then dispose of anyone that's left. There's an HEV power dispenser just inside the room. Press the button to activate the elevator, but don't get on. Look down. Blue lasers are waiting for you. Toss down a Grenade to take them out, then wait for the elevator, press the button again, and get on.