An underground rapid rail transit system or the tunnel through which it operates. The term is sometimes used for the entire rail rapid transit system of which only a portion may be beneath the ground surface. It is an electric railway with the capacity for a "heavy volume" of traffic and characterized by exclusive rights-of-way, multi-car trains, high speed and rapid acceleration, sophisticated signaling and high platform loading. (Also known as heavy rail, rapid rail, elevated railway, or metropolitan railway "Metro")