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Is it true that you can hear a train approaching from a great distance by listening to the metal rails? If so, why?

Answer:

It is indeed true. The rails are much denser than the air, so the vibration travels faster than they do in the air, and travel a longer distance. You can experience something similar in a swimming pool, by clicking two pebbles above the water, and then again under water. The denser water carries the vibration much better than air.
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