It all depends on the frame of reference of the observer.
If the observer is in the train, their frame of reference is the train. Relative to them, you’re motionless, whether the train is moving or not.
If the observer is standing by the side of the railway track and the train is moving past them, their frame of reference is the railway track, and you’re in motion relative to them.
If the observer is in another train, their frame of reference is the other train - and depending on whether it is moving at the same speed as your train or not, you might be in motion or relatively stationary.
