Why can you hear a distant train through the rail before through the air?

Because sound travels about fifteen times faster in steel than in air - roughly 5,000 metres a second against 343. Sound is a vibration passing from one particle to the next, and in a solid the particles are packed close and bonded tightly, so the disturbance passes on far more quickly.

It also loses far less energy on the way, which is why a rail carries a sound for a distance the air would have dissipated. The trick is genuinely old and appears in accounts of tracking across the American plains, though the usual film version - an ear to the rail detecting a train miles off - flatters it a little.

Water sits in between, at around 1,500 metres a second, which is why whale song can cross an ocean basin and why sonar works. The general rule holds across materials: sound is fastest in stiff dense solids, and in diamond it manages about 12,000 metres a second. The one thing sound cannot do is cross a vacuum, because there is nothing to pass the vibration along - which is the single most consistently ignored fact in science fiction.