Final answer:
When the master controller handle of a train is moved to an ON position with a 55-pound indication on the duplex gauge, the brakes will likely be disengaged, causing the train to start accelerating due to the engines' motive force, as explained by Newton's Second Law of motion.
Step-by-step explanation:
If the master controller handle of a train is moved to an ON position when the black arrow of the duplex gauge indicates 55 pounds, the train's braking system is likely being disengaged, assuming the gauge refers to brake pipe pressure. In the context of Newton's laws of motion, particularly Newton's Second Law, this reduction in braking force will allow the train to start accelerating if there's an applied motive force.
In accordance with Newton's Second Law, the motion of the train will depend on the forces applied to it. If the train's engines are powered and the brakes are released, the train will begin to accelerate due to the force exerted by its engines. This can also be related to scenarios provided in example questions, like an elevator accelerating and affecting the scale reading due to changes in net force and the normal force acting on it, as well as consequences of forces between electric current-carrying wires.
Therefore, just as a person standing on a bus or a train must adjust their mass distribution when the vehicle accelerates or decelerates, the train itself will respond to accelerations by either moving if stationary or changing its speed if it was initially in motion.