Maglev (derived from magnetic levitation), is a system of train transportation that uses two sets of electromagnets: one set to repel and push the train up off the track, and another set to move the elevated train ahead, taking advantage of the lack of friction. Such trains rise approximately off the track. There are both high speed, intercity maglev systems (over ), and low speed, urban maglev systems () being built and under construction and development. With maglev technology, the train travels along a guideway of electromagnets which control the train's stability and speed. While the propulsion and levitation require no moving parts, the bogies can move in relation to the main body of the vehicle and some technologies require support by retractable wheels at low speeds under . This compares with electric multiple units that may have several dozen parts per bogie. Maglev trains can therefore in some cases be quieter and smoother than conventional trains and have the potential for much higher speeds. Maglev vehicles have set several speed records, and maglev trains can accelerate and decelerate much faster than conventional trains; the only practical limitation is the safety and comfort of the passengers, although wind resistance at very high speeds can cause running costs that are four to five times that of conventional high-speed rail (such as the Tokaido Shinkansen). The power needed for levitation is typically not a large percentage of the overall energy consumption of a high-speed maglev system. Overcoming drag, which makes all open-air land transport more energy intensive at higher speeds, takes the most energy. Vactrain technology has been proposed as a means to overcome this limitation. Maglev systems have been much more expensive to construct than conventional train systems, although the simpler construction of maglev vehicles makes them cheaper to manufacture and maintain. The Shanghai maglev train, also known as the Shanghai Transrapid, has a top speed of . The line is the fastest operational high-speed maglev train, designed to connect Shanghai Pudong International Airport and the outskirts of central Pudong, Shanghai. It covers a distance of in just over 8minutes. For the first time, the launch generated wide public interest and media attention, propelling the popularity of the mode of transportation. It is also the only maglev train in the world that can be considered as high speed. Despite over a century of research and development, there are only six operational