The Tunnel

The Tunnel is the second chapter of the fourth campaign, Blood Harvest. You start in the freight house on the third floor of a repair building for train cars. Note that there are no weapon upgrades in this safe house. You then travel down the stairs to the offices, a window in one of them leads out to a catwalk above a large train shed. Then descend a yellow beam to ground level. In one of the rooms there is a door with a sign that says "Emergency Exit Only, Alarm will Sound". Make sure everyone is in the room and then close the door before opening the alarm door. The emergency exit triggers a swarm, so be prepared(you can also open the door, close the door and go to the large train shed for a combat on open space). The Exit leads to a big room containing big cisterns, there's a staircase that you'll have to climb to a catwalk. The catwalk leads to a window that goes out onto the roof. Then you can jump across to a coal car to find a tunnel with railroad tracks, you are able to walk a large part of these tracks while on top of the railroad. When you are cleared, jump down and you wil see a pile of rubble which you can use to get on to another traincar. When you come to a large fire blocking a tunnel, turn left and go through a large hole in the wall. There is a large pile of debris to get ontop of the traincar barracade. Then rush to a tanker truck and climb the ladder to get on top of it. A hole in the wall on the left leads to the safe house which is on the left side of the tunnel.

Encounter
In the Lower area of the factory that you have to walk through there is an alarmed door that leads through the factory and onto some train tracks. When you open the door the alarm sounds and the Horde comes. Ammo is located next to the door as well as sometimes health.

There is a way to skip the alarm door Cresendo event. You must hit and move the crates directly under the vent in the ceiling, a box must be stacked on another for you to actually jump into the vent.

Afterwards
After defeating the horde here, the survivors emerge outside and enters the tunnel. Many trains have crashed here, creating obstacles the survivors must traverse. After finally passing all the trains the safehouse appears at the end of the tunnel.