Final answer:
The frequencies observed by a person at the side of the tracks change when a train approaches and after it passes due to the Doppler Effect, while the frequency observed by the engineer remains the same as the emitted frequency because there's no relative motion between the horn and the engineer.
Step-by-step explanation:
The question deals with the phenomena of frequency change due to the relative motion of the source and the observer, known as the Doppler Effect.
(a) The frequencies observed by a stationary person at the side of the tracks would vary depending on whether the train is approaching or moving away. As the train approaches, the sound waves get compressed, and the observed frequency is higher than the source frequency. After the train passes and is moving away, the sound waves get stretched, leading to a lower frequency observed than the source frequency.
(b) The frequency observed by the train's engineer would be the same as the emitted frequency. Since there is no relative motion between the source of the sound (the horn) and the engineer, the frequency of the sound for the engineer would not be affected by the Doppler Effect.
The discussion about the shifts in frequency as the train moves towards or away from the stationary person indicates that the shift is significant and can be noticed, with the shift being 17.0 Hz when the train is approaching and 14.0 Hz when it's receding.