Final answer:
The question examines the Doppler effect and relativity in frequencies observed by a stationary observer and an observer moving with the train. A stationary observer will perceive a change in frequency as the train moves, while the train's engineer will not.
Step-by-step explanation:
The question pertains to the observation of light flashes from both a stationary and a moving perspective, which relates to the Doppler effect and principles of relativity in physics. Specifically, it explores how the frequencies of light are perceived by different observers: a stationary one by the tracks (Observer C) and one moving with the train (Passenger B or the train's engineer).
(a) The frequencies observed by a stationary person (Observer C) will be higher as the train approaches due to the compression of the light waves, and lower after the train passes because of the stretching of light waves (Doppler effect).
(b) The frequency observed by the train's engineer, who is moving with the source of the light, will remain unchanged since both the observer and the light source are in the same frame of reference.