Final answer:
When a train operator hears one long blast of the tower whistle, it indicates that the train is about to move forward.
Step-by-step explanation:
When a train operator operating in a yard hears one long blast of the tower whistle, it means that the train is about to move forward. This is a signal for the operator to be ready to start the train. The long blast is a standard signal used in railroad operations to communicate with train operators.