Your train waits at the platform. Through the window, the train alongside begins to slide smoothly past. For a moment you honestly cannot tell: is it moving, or are you?
You've already seen that positions are measured within a chosen frame of reference. Whether something is moving is a frame question too.
Take a crate on the deck of a boat gliding along a river. In the boat's frame, the crate's position, measured from the boat, is not changing: at rest. In the riverbank's frame, the crate's position changes as the boat glides: moving.
Both answers are correct. There is no frame-free answer to 'is it moving?'. The question only makes sense once a frame is named.
A backpack lies on a seat of a bus driving along a straight road. Which description of the backpack is correct?
A passenger stands still on a moving airport walkway. Which description of the passenger is correct?
The window moment works the same way: name the frame first. Say it was your train that pulled away. In the platform's frame your train is moving; in your train's frame, the other one is.
A woman stands on a station platform as a train rolls past her. Which description of the woman is correct?
Car A and car B drive side by side at the same steady speed down a straight road. Which description of car B is correct?
A cyclist rides past a van parked at the curb. Which description of the van is correct?
A seatbelt buckle rests in its holder in a car driving down a highway. Is there one true, frame-free answer to whether the buckle is moving?