A passenger train consists of one or more locomotives and (usually) several coaches. Alternatively, a train may consist entirely of passenger-carrying coaches, some or all of which are powered; this is known as a "multiple unit". In many parts of the world, particularly the Far East and Europe, high-speed rail is used extensively for passenger travel. Freight trains consist of cars, wagons or trucks rather than carriages, though some parcel and mail trains (especially Travelling Post Offices) appear outwardly to be more like passenger trains. Trains can also have "mixed consist", with both passenger accommodation and freight vehicles. These mixed trains are most likely to be used for services that run infrequently, where the provision of separate passenger and freight trains would not be cost-effective, but the disparate needs of passengers and freight means that this is avoided where possible. Special trains are also used for track maintenance; in some places, this is called "maintenance of way".