Final answer:
When a train operator sounds three short blasts on the train whistle, he is typically indicating that the train is about to move in reverse.
Step-by-step explanation:
When a train operator sounds three short blasts on the train whistle, he is typically indicating that the train is about to move in reverse.
This signal is commonly used in train operations to warn passengers and workers that the train is going to start moving backwards.
For example, a train operator may sound the three short blasts before backing the train out of a station platform.