Final answer:
Train operators must approach certain areas with caution and be able to stop the train if necessary.
Step-by-step explanation:
The phrase train under full control in this context means that the train operator must approach certain areas with caution and be able to stop the train if necessary. This rule is similar to the concept of stopping at a red traffic light for vehicles. Just as vehicles are required to stop at a red light for safety reasons, train operators must approach stop signals, trains ahead, junctions, and terminals with caution and the ability to bring the train to a complete stop if needed.