Final answer:
The question seems to pertain to train operations and the importance of understanding signals, including auditory ones like horns affected by the Doppler effect. Train operators need to interpret these signals correctly to ensure safe travel. Road signs and train horn signals provide necessary information to the operator.
Step-by-step explanation:
The provided excerpts do not directly mention a specific sign that a train operator must follow, yet there is a discussion about train operations and signaling that can be addressed. In the context of physics and train operations, the train engineer must be aware of various signals, including auditory signals like horns and visual signals like road signs. Particularly, the excerpt references a situation involving the Doppler effect, which is a change in frequency or wavelength of a wave in relation to an observer who is moving relative to the wave source. This principle applies to train horns as perceived by the engineer.
For example, if the train operator hears a horn while moving, they need to consider the Doppler effect to accurately gauge the location and motion of other trains which might be represented by auditory signals they are receiving. The given mathematical expression, fo = fs, implies that when the observer and the source of the sound are in the same moving vehicle, the observed frequency (fo) is the same as the source frequency (fs), since their velocities are equal (Us = Vo).
Roadway signals, such as tollbooth stations, mentioned in the query often use sensory mechanisms to alert or notify operators. While not directly related to train signs, it emphasizes the importance of varied signs and signals in transportation systems.