A stationary person hears the pitch of the train's sound increase as it approaches and decrease as it passes, due to the Doppler Effect. The engineer on the train hears a consistent pitch because there is no relative motion between them and the sound source.
The question pertains to the Doppler Effect, which is observed when a sound source moves relative to an observer. A stationary person at the side of the tracks hears a higher frequency as the train approaches due to the compression of sound waves, and a lower frequency after it passes because of their elongation. However, the train's engineer, who is moving with the train, observes a constant frequency because they are stationary relative to the sound source.
In conclusion, frequency changes perceived by the stationary observer are attributable to relative motion, while the engineer's perception remains unchanged. Understanding the Doppler Effect helps explain real-world phenomena like the change in pitch of passing sirens or railroad crossing alarms.