How does the law of inertia relate to passengers on a train that is slowing down suddenly.

Inertia is the tendency of an object to remain at rest or in motion.

According to Newton's First Law of Motion states that an body at rest will remain at rest or move at a constant speed in a straight line unless it is acted on by an unbalanced force.

The train is this case is moving forward it will continue to move forward unless friction or the brakes interfere with its movement. Say for instance the train abruptly accelerated then the passengers may feel it as though their bodies are moving backward. In reality, inertia is making the body want to stay in place as the train moves forward.

Sorry if the explainataion is long. Do you understand or are you confused?