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 train wheels screeching and 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.

While following the tracks, you will come across a cabin where there is ammo, a first-aid cabinet, and sometimes tier 2 weapons. There are two doors in this cabin - one in front and the other at the back. Have everyone camp here, making sure to stock up on ammunition and health if needed. Then TWO (not one) Survivors should go - one to release the train car and knock down the wooden bridge, and the other to stand guard in case there are still Special Infected around (especially Hunters and Smokers). After that, the two MUST make a run back to the cabin and close the door. This way, when the Horde comes, the two doors will funnel and slow down the Infected, making them easy targets for your weapons. However, watch out for Smokers; if one of your teammates is being pulled by one through the windows, shove them right away before he or she takes any damage. Also, be on the lookout for Hunters; they can get in by way of the open windows, so kill them quickly. Another popular tactic is for everyone to wait by the bridge while the train car is released and from there to make a dash to the safe room. This usually works best if The Survivors have a pipe bomb or molotov cocktail on hand.

You can also just guard the top of the boxcar. There are molotovs there and the infected have to climb up the ladder, making them easy targets.

Trivia
When starting the Crescendo event the train car detaches from the one behind it. But if you notice, the link hand phases through the hand of the car behind it.