Final answer:
On sections of track not regularly used, a trackman should perform all of: track inspection, maintenance, and obstacle clearance. The variation in frequency observed by someone beside the tracks is due to the Doppler effect, with no frequency shift for the train's engineer.
Step-by-step explanation:
On sections of track not used for regular passenger trains, a trackman should inspect the track regularly for any defects, perform maintenance work on the track, and ensure that the track is clear of any obstacles. Hence, the correct answer to the student's question is all of the above. This comprehensive approach helps to prevent accidents and ensures the safe transit of any trains that might need to use the track, even if it is not frequently used by passenger trains.
When considering the question related to the frequencies observed by someone beside the tracks and the train's engineer, we're dealing with a phenomenon known as the Doppler effect. A stationary person at the side of the tracks would observe a higher frequency as the train approaches and a lower frequency after it passes, due to the sound waves being compressed as the source approaches and then stretched as it moves away.
The train's engineer, traveling on the train, would not experience this shift and would hear the actual frequency of the train's sounds, because the source of the sound (the train) and the observer (the engineer) are moving together.