Dear belMarduk: Many things can influence such sounds. Wind, temperature, humidity, and foliage. The railroad in my home town was in a shallow valley, and there was a wide area with no obstructions between tracks and my home.
I had leaned against a auto door with faulty latch while auto was crossing tracks, and fell out into path of oncoming locomotive, which fortunately was going slow enough my father was able to stop the car and run back and get me, but locomotive was very close, and I was as you can imagine badly frightened, at about age two and a half.