You proceed along the train trestles, only to find the tunnel lead to the loving horrible town of City 17.
You get out of your Buggy, seeing as how there is a crashed train.
Proceeding up some stairs, you come upon a small building, and decide to enter it through one of the doors.
Coming upon a soda machine, you decide to buy a soda. Instead, the door opens, and you enter a lab.
They notice there's a civilian after Gordon and Alyx teleport back to the lab from the Combine Prison.

"A Civilian has found their way in, well then, good. They should be able to help us in this war, then..." Kliener says.
You hold your shotgun out, and they begin to discuss stuff.
Should you join them, or proceed to a captured train station and escape?
PURPLE = EQUIPPED. Inventory:
WEAPONS Black Silenced Uzi, 9mm Pistol,
Spas-12 Shotgun, Shotgun Ammo, Empty Boomer Bile Jar, Arms, Medkit.
CLOTHINGnational socialist Pig Suit,
Citizen Clothing, Civil Protection Suit, Rebel Armor.
HATS Bonk Helmet.
What should you do?
There is an EASTER EGG in this picture! It's RIGHT IN FRONT OF YOU! :3
It's even mentioned in the text acting.