This does not look like the continuation of the water engine question.

this is another question cause I finished the other part of that question alrdy..I couldn’t find part a of that question that’s why...

Not sure if this needs part one or two results.

But what I am thinking is this.

If a bag is at the BACK at of the carriage, and the train abruptly stops, the bag will not be able to react to the slowing down in time, so it will still be moving forward.

As a result, relative to the train, the bag still moves forward, so the bag will fly to the front.

In any case, the bag will fly towards the front of the train.

Even if the bag is in front, the inertia will cause the bag to not be able to react to the stoppage in time, so the bag will still be moving forward when the train stops.

It cannot be the case that the bag ever flew backward.

Imagine you have a cart of sand and you intentionally crash the cart against the wall. Which direction will the sand go?

It's the same idea.

ohh ok I get it...thanks for your help