Final answer:
A stationary person observes higher frequencies as the train approaches and lower frequencies after it passes, due to the Doppler Effect. The train's engineer will hear the actual frequency since they move with the sound source and do not experience this effect.
Step-by-step explanation:
The query relates to the Doppler Effect, which is a phenomenon observed when a source of sound moves relative to an observer, affecting the frequency of the sound waves received. Two different scenarios are described in this question:
- (a) A stationary person at the side of the tracks will observe a higher frequency as the train approaches, due to the sound waves being compressed. As the train passes and moves away, the frequency becomes lower because the sound waves are stretched out.
- (b) The train's engineer, however, who is moving with the train, will observe the actual frequency of the sound being emitted since they are in the same reference frame as the sound source. There is no Doppler Effect relative to the engineer's position.
This effect is crucial for train operators when power is restored to a third rail; they must be attentive to the acoustic signals which may be altered due to their motion relative to the stationary sources (e.g., signals and warnings on the track side).