Why can you hear a train approaching when you put your ear on the railway track?
Sound travels efficiently through solid steel rails to your ear
Sound is a mechanical wave: it needs a material medium (solid, liquid or gas) to propagate, because it travels by the vibration of the medium's particles. The speed and efficiency of that propagation depend on how tightly the particles are packed and how strongly they interact.
In solids, particles are packed very close together and are strongly bonded, so a vibration set up at one end is passed along quickly with little loss of energy. That is why the speed of sound in steel (around 5000 m/s) is much greater than in air (around 340 m/s), and the sound also attenuates far less over long distances in the rail.
When you press your ear against a railway track, the vibrations produced by a distant approaching train travel through the continuous steel rails and reach your ear well before the same sound arrives through air.
Hence, you can hear the train because sound travels efficiently through the solid steel rails to your ear.
The sound that reaches you through air is much weaker and slower, so that alone would not let you detect a still-distant train. It is not that the rails somehow vibrate more loudly than air; they simply transmit the same vibrations more efficiently. And the vibrations are not restricted to the surface of the rails; they travel through the bulk of the steel as elastic waves.
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