An old trick to find out if a train is coming is to put your ear on the rails of the track. Why is this more effective than just listening for the sounds the train makes in air?

Answer 1

Soundwaves in the air will have spread out over a larger and larger area the farther they travel. Sound waves that travel through rails travel in one dimension, they don't spread out.

The the reaosn why sound waves in the rails wll stay in the rails, is because the velocity of sound in rails is much higher than the velocity of sound in the ground and in the air. This causes most of the sound waves to reflect off the boundary between the rails and the ground or air and stay in the rails.