Final answer:
The sound from a train is higher in pitch for a stationary observer when the train is approaching due to the Doppler effect. The frequency heard by the observer changes based on whether the train is moving towards or away from them, while the passenger on the train experiences a constant frequency.
Step-by-step explanation:
The sound from the train, as heard by an observer, is higher in pitch than the sound heard by a passenger on the train due to the Doppler effect. As the train approaches an observer, the sound waves are compressed, leading to higher frequency sound waves and thus a higher pitch. Conversely, for a passenger on the train, the frequency of the sound waves remains unchanged because both the source of the sound (the train's whistle) and the listener (the passenger) are moving at the same speed. This effect is reversed as the train moves away from the observer, at which point the observer hears a lower pitch as the sound waves are stretched out, decreasing in frequency.
When considering the change in frequency due to the Doppler effect, one may use the equation with a plus sign to find the frequency heard by a stationary observer as the source moves towards them and the minus sign as it recedes. The observed frequency will be higher when the source (train) is approaching the observer and lower when it is moving away. The actual frequency aboard the train would be experienced by the train's engineer without the effects of relative motion.