Each concept here is mapped to its prerequisites — the ideas you'd need first to understand it — all the way down to four foundations: Space, Time, Energy, Pattern. Click any prerequisite to drill down, or scroll for the chain graph.
A metro is a rapid transit system consisting of underground rail lines and stations serving urban areas, characterized by frequent and high-capacity transportation, but this definition lacks a clear understanding of mass transit.
External reference — all senses of the word “metro” on Wiktionary. This atlas concept maps to only the slice of meaning relevant to the prerequisite graph.