The word 地铁 is formed by combining the character for "earth" or "ground" (地) with the character for "iron" (铁). It creates a new, compound concept through a direct, literal image: the "iron" (representing the railway or train) that is associated with the "ground". This specifically evokes the system of trains running on tracks, often underground, thus forming the meaning "subway" or "metro" through this conceptual blend of place and material.