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Outside Switzerland the first locomotive to run on compressed air was built in 1890, and by 1895, the basic principles of efficient compressed air engines had been developed. A particularly important engineering breakthrough was the development of the reducing and stop valve which maintains a uniform pressure of air to the engine, even as the pressure in the storage tank reduces with use. Compressed air locomotives have been used for many years, mainly in mines, but have also been used on tramways. (See Mekarski system)