Concepts groups the foundational ideas that explain how transport actually works and changes over time, containerization, standard gauge, logistics and induced demand among them, organized into rail, road, maritime, aviation, logistics and intermodal, transport planning and policy, legal and economic, transit and systems operations, and social and equity concepts. A concept is an idea or practice rather than a physical thing, an organization or a place. This category excludes the vehicles a concept might describe, which belong under Vehicles, the companies that put a concept into practice, which belong under Networks and Operators, and the built infrastructure a concept might govern, which belongs under Infrastructure.