A passenger train is one which includes passenger-carrying vehicles which can often be very long and fast. One notable and growing long-distance train category is high-speed rail. In order to achieve much faster operation over 500 km/h (310 mph), innovative Maglev technology has been researched for years. In most countries, such as the United Kingdom, the distinction between a tramway and a railway is precise and defined in law. The term light rail is sometimes used for a modern tram system, but it may also mean an intermediate form between a tram and a train, similar to a subway except that it may have level crossings. A freight train uses freight cars – essentially any train that is not used for carrying passengers.