What additional signal must a train operator have from the conductor before starting a train carrying customers from a terminal?

The necessary additional signal that a train operator must receive from the conductor before starting a train carrying customers from a terminal is a verbal confirmation. This ensures clear communication between the conductor and the operator, confirming that all safety checks and procedures have been completed and that it is safe to depart. Verbal confirmation is vital in maintaining safety protocols, as it provides an opportunity for the conductor to communicate any pertinent information about the train or the loading process that may not be conveyed through other signals.

The other options do not provide the level of clarity and assurance needed to ensure a safe departure. A single horn blast could be misinterpreted or not clearly understood, particularly in a busy terminal environment. Two long buzzer signals might also lack clarity, as it not only requires interpretation but may vary between operations. A visual signal from the window might rely on visibility conditions, which could change rapidly, leading to potential misunderstandings. Thus, verbal confirmation stands out as the most reliable and effective means of communication in this context.