A marble is at rest in the aisle of a train car. Qualitatively predict the displacememt of the marble relative to the smooth, horizontal floor of the car as the train enters and leaves a station.

I'm struggling on conceptualizing this because I feel like it should be at equilibrium in a train car system as per the frame of refs, but that's only if the external system is in constant motion... but this isn't. If someone has a great way to word it thank you so much!!!