Science Puzzle
Why Can You Hear a Train Through the Rails First?
In old films, someone presses an ear to a railway line and hears a train coming long before it can be heard through the air. It really works, but don't ever try it: trains are faster and quieter than they look.
Why does the sound arrive through the rails first?
The Answer
Sound travels much faster through steel than through air, about 5,900 metres a second compared with about 343. And the rail guides the sound along it, so it stays strong instead of spreading out in every direction.
Sound is a vibration passed from particle to particle. In air, the molecules are far apart and only bump into each other now and then, so the vibration is handed on fairly slowly. In steel, the atoms are packed tightly together and held by strong bonds, so a push on one is passed to the next almost at once. That's what makes steel so stiff, and sound races through it.
For a train 2 kilometres away, the sound takes about 6 seconds to arrive through the air, but only about a third of a second through the rail. Sound in the air also spreads out in all directions and fades quickly, while the rail keeps the vibration channelled along its length.
Sound travels faster through water than air too, about 1,500 metres a second, which is one reason whales can hear each other across huge distances.
The principle: The speed of sound depends on the material. Sound travels much faster through stiff solids like steel than through air, and a rail guides it along without letting it spread out.