After starting the chapter, go under the bridge and look for a secret gadget (1/16) on the support. Follow the marker. At the first spacious location where the kamikaze will appear, look for a passage between the rocks on the left. It is hidden by bushes. This path leads to a Secret Hut on Chicken legs (2/16). Go past the church, go down to the shed with the tank and climb onto the roof to find a secret gadget (3/16). Move on the tank through a large location. Take your time, because there are really a lot of enemies, and the tank has a small margin of safety. Along the way, you need to destroy two turrets. After the tank, when Sam is forced to continue on foot, go forward and turn left behind the house. Look for boards. Break them and take secret ammo (4/16).
Once you reach the marker, you can start the side quest. Listen to the flying drone and follow the marker. Go inside the tunnel, kill the first enemies and in the corridor where the enemies in orange suits stand, enter the room on the right. Break the boxes and go downstairs. Set all 8 levers to green to get a secret automatic shotgun (5/16). Keep moving. The marker points to the pipe. You pass by it forward along the corridor and call the elevator. A secret army of Headless (6/16) will appear. Keep moving until you go down the big elevator (you need to kill the enemies). Go even further down the corridor, kill the skeletons. There will be a fence on the right. Jump over it with the spotlight and find the ammo cache (7/16). Get to the portal, move through it and destroy the two bosses.
Return to the surface and follow the marker until you meet a squad of Russians in Kalinovka. After the video, play the guitar a few times until you get God's secret guitar (8/16). In the neighboring courtyard, there will be a new weapon (burner), and behind the house on the right - secret armor (9/16). Go through several yards using the new weapons until you reach the train (behind the hole in the fence). Go along it to the right and look for an ammunition cache (10/16) behind the stones. Keep walking until you see a small door ahead that says BUILDING 1. It's not that far from the train. Opposite the door there are destroyed metal steps. From the remaining pieces, jump onto the roof and take the secret gadget (11/16).
A little further, in the neighboring courtyard, kill the enemies and pay attention to the grate on the right. Go around the building and jump over the truck to find the secret gadget (12/16) behind the crates. The marker will point to the building, but you need to go forward along the rails and find a wall in the building on the right, which you need to blow up with explosives. Do this and find the secret classic level (13/16). After passing it, go around the same building and find a secret gadget (14/16) in the corner. Follow next. When you leave the building where you found the grenade launcher and its upgrade, look for secret armor (15/16) behind the crates on the left. In a spacious area with rails where you need to protect the train, go to the far-left corner and look for an ammo cache (16/16) behind the building. Kill the enemies, move the containers aside, as you did in the second chapter, continue to fight, and soon you will be able to leave the station.