Left 4 Dead/The Bridge

You've passed through the various train tracks.

Route

 * Take a left - it's fastest way to the side passage.
 * The side passage is blocked off, but has an exit to the left.
 * Head down the valley. At the lamp, you're turning right and going up the hill.
 * Enter the building.
 * After going downstairs, head straight forward to the tracks and turn left onto them.
 * Follow the tracks to the train car.
 * Once there, decouple the train car - the squeaky wheels is loud enough to alert the horde, but having the train car knock down the bridge is just as noisy.
 * Climb the fallen bridge, and run down the path to the safe house.