Final answer:
The least important information for a train operator to report when reporting a fire is the color of the train.
Step-by-step explanation:
The train operator's main priority when reporting a fire by phone is to ensure the safety of passengers and crew members. While all the information provided is important, the LEAST important for them to report would be the color of the train.
This is because the color of the train does not directly affect the response or handling of a fire incident. On the other hand, the location of the fire is crucial for emergency personnel to quickly respond and locate the incident, the size of the fire allows for appropriate resources and equipment to be dispatched, and the type of fire extinguisher used provides valuable information to respond effectively.