Final answer:
The first action to bring the train back under control after the motors continue to take power is to engage the emergency brake.
Step-by-step explanation:
If the motors continue to take power after the train operator of a passenger train returned the master controller to OFF, the first action in bringing the train back under control should be to engage the emergency brake.
The emergency brake is designed to quickly and effectively stop the train by applying a strong braking force to all wheels.
By engaging the emergency brake, the train operator can regain control over the train's movement and bring it to a stop.