Final answer:
The train operator's best initial action is to evacuate the passengers.
Step-by-step explanation:
The best initial action for the train operator to take when noticing considerable arcing underneath the car is to evacuate the passengers. Arcing can be a sign of an electrical malfunction or a potential fire hazard. It is essential to prioritize the safety of the passengers and remove them from the train as a precautionary measure.
Calling for maintenance would be the next appropriate step after evacuating the passengers. Ignoring the arcing or continuing to operate the train could put the passengers and the train at greater risk.