Why is the sound of an oncoming train different from a train that's moving away?

Answer 1

This is due in part to your reticular activating system - part of your brain - which alerts you to things (audio & visual) that may be dangerous. (It also does other things, too!)

Your reticular activating system is listening out for dangerous sounds, and recognises an oncoming train as being potentially dangerous. Once the danger has passed, and nothing bad has happened, your reticular activating system is 'un-switched', and your brain doesn't need to hear it anymore, hence trains sound different moving away.

The same is true of emergency vehicle sirens - if you hear them, you are alerted to them by your reticular activating system, and once they have passed and are no longer a danger, your brain doesn't need to register the siren anymore.

I am doubtful but not dismissive of the information above regarding the reticular activating system. Whether or not there is any truth to it, the questioner may be making reference to the Doppler Effect, which has to do with the frequency of the sound waves hitting the ear when a train is approaching in comparison to the frequency when the train is moving away. While the sound waves themselves are moving at the same speed through the air no matter where the train is, the signal waves coming toward us as the train is advancing are compressed, and as a result we will perceive them as being at a higher pitch than when the train is receding, and the waves are less compressed.

Its entirely the Doppler Effect and not the brain. The brain has no way to know if a sound is approaching or receding to then make it appear to sound different. Due to the Doppler Effect the actual frequency of approaching and receding sounds is different when they arrive at the ear as the waves are compressed and expanded, respectively.