Final answer:
The observation of a motionless ball hanging on a string inside a moving train implies that the train is either stationary or moving at a constant speed. The correct inference, given the context of the question, is that the train is moving at a constant velocity on a horizontal track.
Step-by-step explanation:
If an observer inside a train car sees a ball hanging motionless from a string tied to the ceiling, then we can infer that the train is moving at a constant speed or it is stationary. However, given that we are presenting a scenario where the train is on a horizontal track and the ball is not swaying, the most sensible inference is that the train is traveling at a constant velocity. This is because, in a non-accelerating frame of reference (which is a train moving at constant velocity), objects will behave as if they were in a stationary frame—thus the ball hangs motionless.
When the train accelerates or decelerates, the inertia of the ball would cause it to swing backward or forward, respectively, from the perspective of someone inside the train, due to Newton's first law of motion. This concept is also related to inertial frames of reference, in which the laws of physics operate in the same way as in a stationary frame when no net force acts upon an object.