Part 21

As the lift reaches the top, have the assault rifle ready to quickly eliminate the marine waiting there. A previously placed tripmine might catch this marine. The second tripmine should take out another marine approaching.

The explosion from the tripmines will open a headcrab vent. Deal with the headcrabs or run past them.

To deal with the third marine, go past the HEV station to a small red-railed balcony. Strafe onto the balcony while facing right, then immediately strafe back out of sight. The marine should trigger the first tripmine and be eliminated.

Check for any needed supplies and use the HEV station if necessary before returning the way you came.

After passing the underground bunker and moving down the hall, you will return 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, be prepared to shoot a vortigaunt with the assault rifle.

Continue until you reach a room with a ladder. Climb up and use the crowbar to break the boards and boxes blocking the door to the track control room.

Enter the track control room, note the HEV station on the left wall, and approach the injured guard.

Take the .357 Magnum from next to the guard.

Move near the large window to ensure the gargantua approaches it.

Leave the room and jump over the large gap in the walkway.

Carefully proceed past the flaming pipe and reenter the large chamber where the gargantua is located.

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

Defeating the Gargantua (Grenade Method): Keep your distance and toss hand grenades between its feet. If you run out of hand grenades, use AR grenades or return to the power generator area for more hand grenades from the underground bunker room (watch out for a new vortigaunt on the way). Quicksave frequently and quickload after misses, as the gargantua moves erratically.

Defeating the Gargantua (Power Coil Method): Lead the gargantua to the large power coil room. Run through the doorway, turn left, and hug the left wall down the large tunnel. The gargantua may not follow. If it does, run for the power coils' activation switch. Jump up to hit the switch from the corner on the right past the coils. Stay in the corner until the gargantua is destroyed. It's easier to lure the gargantua if you wait for it to move away down the rail tunnel. If it doesn't follow, try returning to section c2a1b and coming back, or wait until its back is turned.

If the gargantua doesn't follow you down the tunnel, enter the coil room, cross it, and go up the steps. Cross the raised walkway to find the switch. After activating the coils, go back down and through the doorway on the right.

In the small service tunnel, push the door open. Go to the start of the large tunnel and move around until the gargantua enters, then quickly run back through the door you just opened. Wait in the doorway until the gargantua is destroyed by the power coils. If it doesn't get destroyed, you may need to go up the steps and let it see you on the raised walkway.

Break open crates near a power coil to find 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 large window and use the switch to activate the mechanism the rail car is on. 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 riding the car to reach the "On a Rail" chapter.