Final answer:
The stationary person hears varying frequencies due to the Doppler effect, with higher frequencies as the train approaches and lower after it passes. The train's engineer hears a constant frequency because the source and observer are moving together.
Step-by-step explanation:
The question pertains to the Doppler effect, which explains how the frequency of sound waves changes due to the relative motion of the source and the observer. Frequencies observed by a stationary person at the side of the tracks will be higher as the train approaches and lower after the train passes because of the Doppler effect. This is because the sound waves are compressed as the train approaches and stretched as the train moves away. On the other hand, the frequency observed by the train's engineer traveling on the train does not change due to the relative motion since both the source of the sound (the train's whistle, for example) and the observer (the engineer) are moving together.