Final answer:
Cowboys listening to train tracks benefited from the efficient travel of sound through solids, which enabled them to detect trains from greater distances than through air. The Doppler effect, observing the change in wave frequency relative to a moving observer, is also relevant when describing the change in pitch of sounds like a train whistle as the train moves.
Step-by-step explanation:
Cowboys in old western movies would often put their ear to a train track to listen for an oncoming train. The benefit this method provides over listening for a train through the air is that sound waves travel more efficiently through solids than they do through gases. In a solid medium like a train track, sound travels faster and with less energy loss compared to air. Therefore, the vibrations from an approaching train can be detected from a greater distance when listening through the track.
When discussing sounds from a moving source, the Doppler effect also comes into play. The Doppler effect describes the change in frequency of a wave in relation to an observer who is moving relative to the wave source. This is what occurs with a train whistle or a police siren as it approaches and then moves away from you, resulting in a change in the pitch of the sound waves.
To appreciate Christian Doppler's observation on the effect that bears his name, it's essential to understand that he observed musicians both on a moving train and from a stationary point. This was to gather evidence of how the frequency of sound waves changes due to the relative motion between the source and the observer. When a train approaches, the frequency of the sound waves is higher as they are compressed; as it moves away, the frequency decreases as the waves are stretched.