Subcategories groups the finer-grained anchor entities minted beneath each category, such as Diesel and Electric Locomotives beneath Vehicles or Rail Mode beneath Modes. Each subcategory anchor carries prose describing what belongs in that narrower grouping and lets a visitor browse a manageable slice of a large type rather than one long undivided list. Like Categories, this is a structural type serving the atlas's own organization rather than a transport subject in its own right, and excludes any entity describing an actual vehicle, mode, figure, concept, network, route or piece of infrastructure, which belongs under its own driver type.