Final answer:
The excerpts provided discuss various scenarios involving trains but do not give clear instructions regarding what a train operator should do at a specific sign. The context of the sign is missing, and the provided text includes discussions on the Doppler effect and children placing objects on a train track.
Step-by-step explanation:
The question you asked appears to be related to train motion and its observational consequences.
However, the excerpt provided seems somewhat disconnected, talking about various situations involving trains, such as a character putting items on a track and a passage discussing an observation of relative speeds, which appears to involve the Doppler effect.
Specifically, the latter part mentions the observation and the propagation speed of sound relative to the observer and the source. If the question is about what a train operator should do at a given sign, there is no direct information provided about the sign, nor is it clear what the instructions are for the operator that pertain to that sign.
As for the physics concepts mentioned, if the content refers to the motion of a train and observer effects such as the Doppler effect, it would be important to know the contexts, such as whether the sign is about speed limits, station approach, or crossing notifications.