in China are referred to as "Light Rail" but qualify as rapid transit because they are fully grade-separated and provide a high frequency of service. Many cities use names such as subway and elevated railway to describe their entire systems, even if they combine both methods of operation. Slightly less than half of the London Underground's tracks, for example, are actually underground. The New York City Subway also combines elevated and subterranean stations, and the Chicago "L" and the Vancouver SkyTrain use tunnels to run through central areas. Other types of services Guided bus A bus shares many characteristics with light rail and trams but does not run on rails. Trolleybuses are buses that are powered from overhead wires. Vehicles that can travel both on rails and on roads have been tried experimentally but are not in common use. The term bus rapid transit is used to refer to various methods of providing faster bus services and the systems that use it have similar characteristics to light rail. Guided buses are buses capable of being steered by external means, usually on a dedicated track or roll way that excludes other traffic. Some cities experimenting with guided bus technologies, such as Nancy, have chosen to refer to them as 'trams on tires' (rubber-tired trams) and given them tram-like appearances. Economics In a 2006 article, the political scientist Ted Balaker and the urban planner Cecilia Juong Kim stated that public rail transit provides certain benefits for a community but also that the goals of policymakers are not often met. They also note some American economists claim that contrary to popular belief, rail transit has failed to improve the environment, serve the poor, or reduce highway congestion in the United States. They also claim economists are somewhat more optimistic about rail transit's impact on economic development. See also Passenger rail terminology List of tram and light rail transit systems List of town tramway systems Streetcars in North America List of North American light rail systems by ridership Medium-capacity rail system List of premetro systems List of metro systems List of automated train systems List of North American rapid transit systems by ridership List of rail transit systems in North America List of monorail systems List of suburban and commuter rail systems List of United States commuter rail systems by ridership List of funicular railways List of bus rapid transit systems List of United