The sound of the distant train can be heard on the line, but if you take your ear off the line, why can’t the sound be heard?
Answer: If you listen to the railway line, you can hear the sound of distant trains. But if you take your ear off the line, you can no longer hear the sound of the train. Sound travels through iron, so if you listen to the train, you can hear the sound of the train.
If you remove the ear from the line and the ear is in the air. Since the density difference between iron and air is large, sound cannot propagate from iron to air. So if you lift your ears from the line, you cannot hear the sound of the train.