Part 21

When the lift reaches the top, be ready with the assault rifle to shoot a marine near it. A tripmine you placed earlier might kill him. Another tripmine should take out a second marine.

The explosion may open a headcrab vent; deal with the headcrabs or run past them to the final marine.

To reach the third marine, go past the HEV station to a small red-railed balcony. Strafe onto it, then immediately strafe back out of sight. This marine should trigger your first tripmine.

Check for supplies and use the HEV station if needed. Return the way you came, past the underground bunker and down the hall to section c2a1b.

Section c2a1b (again): Return to the room with the first aid station and climb down the two ladders in the shaft. Go through the open doors and down the hall to where section c2a1 reloads.

Section c2a1 (again): As you turn left in the hall, shoot a vortigaunt with the assault rifle. Continue to the room with a ladder.

Climb the ladder and use the crowbar to break the boards and boxes blocking the door to the track control room.

Enter the track control room, noting the HEV station, and go to the injured guard.

Take the .357 Magnum from next to the guard. Move near the big window to ensure the gargantua approaches it.

Leave the room and jump over the gap in the walkway. Carefully pass the flaming pipe and reenter the large chamber with the gargantua.

The gargantua will likely see you and attack with super-heated steam.

Keep your distance and toss hand grenades between the gargantua's feet.

If you run out of hand grenades, use AR grenades.

If you still need more grenades, return to the power generator area and get them from the underground bunker room near where you fought marines. Watch out for a new vortigaunt on the way.

Quicksave after each successful hit and quickrestore after misses, as the gargantua moves constantly. Be patient.

Lead the gargantua to the big power coil room.

Before it senses you, run through the doorway, turn left, and run down the large tunnel, hugging the left wall. The gargantua may not follow if you stick to the wall.

If it follows, run for the power coils' activation switch. You can reach the higher floor by jumping from the corner past the coils. Stay in that corner until the gargantua is destroyed.

To make it down the tunnel without being followed, wait for the gargantua to move away down the rail tunnel. If it doesn't, return to section c2a1b and come back, staying out of sight. Alternatively, wait until it's on the opposite side with its back turned, then run for the switch.

If it doesn't follow you down the tunnel, turn left at the end to enter the coil room. Cross it and go up the steps.

Cross the raised walkway to find the switch. Activate it.

Go back down the steps and through the doorway on the right.

At the end of the small service tunnel, push the door open.

Go to the start of the large tunnel and move around until the gargantua enters the tunnel after you. Quickly run back through the door you just opened.

Stand in the doorway at the other end of the service tunnel until the gargantua is destroyed by the power coils. If it's not destroyed, go up the steps and let it see you on the raised walkway.

Break open crates near a power coil for hand grenades.

Return to the gargantua's chamber and get on the rail car. Move it forward as far as possible.

Return to the track control room. Use the HEV station if your armor is below 100%.

Go to the big window and use the switch to activate the rail car's mechanism. Look for ammo on a control panel to the right of the window.

Jump through the window to shortcut back to the rail car. Get on it and start it moving.

After crashing through a barricade, stop the car and use the flashlight to find health packs to the right of the track (these may not always be present).

Continue on the rail car to reach the "On a Rail" chapter.