Final answer:
A train operator typically uses two short blasts of the horn to indicate movement or to alert for safety reasons. The meaning of the horn signals can vary by region. The Doppler effect explains the change in pitch of the whistle as trains move.
Step-by-step explanation:
When a train operator sounds two short blasts of the horn or whistle, it usually signifies certain actions or signals as required by railroad regulations.
The specific meaning can vary depending on the country and the railroad company's rules. However, common reasons for this signal include indicating that the train is starting to move, a warning that the train is approaching a crossing, or an acknowledgment of received signals. It's a form of communication used by train engineers to convey messages to others on the train, workers on the rails, and people nearby.
The sounds we hear, such as a train's whistle, can be affected by the Doppler effect, which is a change in the frequency of a wave in relation to an observer who is moving relative to the wave source. This phenomenon explains why the pitch of a train whistle changes as it approaches and then moves away from an observer, with the pitch sounding higher as it approaches and lower as it moves away.