The dividing line between metro and other modes of public transport, such as light rail and commuter rail, is not always clear, and while UITP only makes distinctions between "metros" and "light rail", the American Public Transportation Association (APTA) and Federal Transit Administration (FTA) distinguish all three modes. A common way to distinguish metro from light rail is by their separation from other traffic. While light rail systems may share roads or have level crossings, a metro system runs, almost always, on a grade-separated exclusive right-of-way, with no access for pedestrians and other traffic. And in contrast to commuter rail or light rail, metro systems are primarily used for transport within a city, and have higher service frequencies and substantially higher passenger volume capacities. Furthermore, most metro systems do not share tracks with freight trains or inter-city rail services. It is however not relevant whether the system runs on steel wheels or rubber tyres, or if the power supply is from a third rail or overhead line.