Wake up and open your eyes. You somehow found yourself alone on the train after the chaos of the Citadel explosion in Episode 1. And by the way, you're alone again! She's gone! Follow the train to the very end and look to the left. Alex will soon run up, exchange a few words, find the Gravity Gun, and open the door. Equip the gun and run to the right of the train, open the door with the right mouse button and go inside. Look at the destroyed city from the rocks, then continue on as the shockwave from the portal sweeps across the area and breaks the rocks blocking the tunnel. The path is linear, a continuation of the tutorial, with a small number of zombies.
Jump off the tunnel into the adit, avoiding all the undead along the way. By starting to run and holding shift after 6 meters from the entrance, you can run through all the doors without waiting for Alex. However, she may linger to kill any surviving zombies. Help Alex get the radio working and wait for her to contact White Forest. Next, enter the next tunnel and jump on the poisonous spiders, letting your partner deal with them. Unblock the cluttered path with the Gravity Gun. Once you drop down to the train station, find the only way to the top of the structure. Go down to the fence control room, open it, and exit through the blocked door.
Poor Alex will be caught by an Alliance Hunter and stepped on, but he will later be killed by a Vortigaunt, who will also help you get back on your feet. Once the area is clear and you've found the elevator location, grab the Crowbar and head down. Collect loot as you make your way through the shaft to the elevator, and grab the ammo on the floor above. Using the Gravity Gun, head to the exit point (keep an eye out for loot). After going several levels deeper, through holes in the floor, you'll finally reach a location with a hole and a visible white light below. If you try hard enough, you can quickly run towards it.
There are several other places you can explore ahead, and running will help you avoid getting stuck. Continuing to run to the left of the tracks, near the mining machines, you'll be attacked by Antlions. Use the cannon to destroy the ammo boxes on the table near the lockers. When using the explosive barrels, face the hole in the wall next to the lock, staying close to the right edge of the wall as you move forward. Release the barrel, and clear the way with the Gravity Gun. Loot the armor from the locker and throw the barrel into the hole as you pass through on the left. Search the lockers on the right again for armor batteries and drop down.
Note: Spider Ants spit acid as an offensive weapon, which deals minor damage to Gordon if you don't get too close. You'll first encounter them behind the grate in the mine.
Source: www.gamefaqs.com.
Chapter 1: To the White Forest (Part 2) [Episode Two]