Imagine you are waiting for a train to pass at a railroad crossing. Will the train whistle have a higher pitch as he train approaches you or after it passes you y?

As the train approaches you, the whistle will have a higher pitch due to the Doppler effect, where sound waves are compressed as the source moves closer. Once the train passes and moves away, the pitch will drop as the sound waves are stretched out. This change in pitch is a common experience when observing moving objects that emit sound.