Rapid transit

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A Rapid transit (or Metro(politan), subway, underground, tube, elevated system) is a railway system. Usually it is located in an urban area. Very often, it runs underground, but parts of it may be above ground. Trains usually run very frequently, and carry a high volume of passengers. Usually it is separated from other traffic by a concept called grade separation. Grade separation is the fact that when two axes of traffic cross each other, they do so on different levels, so as to not influence each other.

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