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 a building where they can find weapons, First aid kits, pipe bombs, molotov cocktails, pain pills or ammo. After following the train tracks they find a cabin where they find ammo and health (pills or first aid kits).

Crescendo Event
The Survivors have have to pull a lever on a boxcar, which uncouples it from the train causing it roll backwards. The train car knocks down a bridge that creates a path they can use to reach the safe house. The noise of the bridge falling down summons 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.