Final answer:
The best action for the train operator is to sound the appropriate signal to warn the conductor and then take additional measures to stop the train.
Step-by-step explanation:
In this scenario, the train operator realizes that the train will not stop in time using normal braking. The best action for the operator is to sound the appropriate signal to warn the conductor and then take additional measures to stop the train, such as applying emergency brakes or using other available means to slow down the train.
By sounding the appropriate signal, the operator notifies the conductor and other relevant personnel about the situation, allowing them to take necessary actions to ensure the safety of the passengers and prevent any potential accidents or injuries.