Final answer:
The frequencies observed by a stationary person will be higher as a train approaches and lower after it passes due to the Doppler Effect, while the frequency observed by the train's engineer remains unchanged. The danger of standing too close to tracks is from lower atmospheric pressure created by rapidly moving trains.
Step-by-step explanation:
The question relates to the phenomenon known as the Doppler Effect, specifically when applied to trains and the sound of their horns as perceived by different observers. The following answers correspond to the posed questions:
- The 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. This is due to the Doppler shift which occurs because the sound waves are compressed as the train moves toward the observer and spread out as it moves away.
- The frequency observed by the train's engineer traveling on the train will be the original frequency emitted by the horn. As there is no relative velocity between the horn and the engineer, no Doppler shift is experienced.
Regarding the danger posed by standing close to railroad tracks, it is attributed to the reduced atmospheric pressure created by the passing train, which can lead to a person being pulled towards the train due to the pressure difference.