Half-Life: Opposing Force/We are Pulling Out

The tram
The tram ride in this section, unlike the tram rides of Fremen and Calhoun, is short and will have an enemy appear and attack you. Shortly after this single encounter, you will approach a platform, where you disembark.

A new contact
Exit the platform to pickup the Glock 17. Shortly after doing so, two aliens will spawn; one on the platform, and one on top of the tram.

Head around the corner, and have Otis follow you through the corridor, killing the headcrabs, zombies and hound eyes. When you see Barney and a scientist talking, a new alien will appear and kill both of them. Jump through the window, and open the door for Otis.

Otis will unlock a door containing a few grenades for an MP5 you'll pick up shortly.

Go on the elevator and press the button to raise it. Dispatch the one or to enemies at the top of the lift. Recharge PCV power as required, and go through the grate on top.

Evacuation
Head through the vent, and climb the ladder to the top. You will reach a military rally point, where a soldier uses a radio, and another tried beating information from a scientist.

The closet containing the scientist and soldier contains a few HEV charge cells. You can collect them either by going through a raised wall vent on the floor below, or by committing friendly-fire by killing the soldier. In either case, proceed down the stairs to the helicopter.

However, the helicopter access is closed and it takes off without you. Instead, the only route out will be through a wall which an alien tries to burst through. Proceed into this hole to start the next chapter.