The Bridge (Left 4 Dead)

The Bridge is the third chapter of the fourth and final campaign, Blood Harvest. The Survivors travel down a tunnel where they go to the woods and must find the train tracks again. They enter on a building where they can find weapons, First aid kits, pipe bombs, molotovs, pain pills or ammo, Following the train tracks they find a cabin where they find ammo and health (pills or first aids)later they pull a lever and breaking the bridge, this will attract the horde, the survivors can fight the horde on open space or in the cabin.

Crescendo Event
You have to pull a lever on a freight of a train, it will roll backwards and knock down a bridge that brings you to a safe house. This alerts a Horde.

Tips
If you are low on ammo and supplies, it is probably recommended to attempt to run with the train to go up the bridge as soon as it falls. The bridge itself makes a good choke point to fight the infected at, and you can quickly reach the saferoom from it. If you have a pipe bomb, you can even assure your rather quick escape from the Horde.