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In railroad terminology, double-heading or double heading indicates the use of two locomotives at the front of a train, each operated individually by its own crew. The practice of triple-heading involves the use of three locomotives. Double-heading is most common with steam locomotives, but is also practised with diesel locomotives. It is not strictly the same practice as two or more diesel or electric locomotives working 'in multiple' (or 'multiple-working'), where both (or all) locomotives are controlled by a single driver in the cab of the leading locomotive. ==Advantages== Double-heading is practised for a number of reasons: * The most common reason is the need for additional motive power when a single locomotive is unable to haul the train due to uphill grades, excessive train weight, or a combination of the two. * Double-heading is also used on passenger trains when one locomotive could suffice but would not be fast enough to maintain the schedule. * More rarely, certain companies have used double-heading to guarantee a service when they have been aware of the poor quality of their locomotives, on the understanding that if one engine failed in service, the other would suffice to get the train to its destination. * Double-heading is a useful practice on single lines even in the absence of a need for more power, as to double-head a train saves making a separate path for a spare engine; it can be repositioned using the traffic path occupied by the service train. * As double-heading has become increasingly uncommon railway companies may advertise specially double-headed services as an attraction to enthusiasts; this occurs regularly but infrequently on the British mainline, whilst the Romney, Hythe & Dymchurch Railway in England advertises an annual day when all of its passenger trains are double-headed all day, both steam and diesel. * In the United Kingdom, double-heading is used to provide redundancy for all trains hauling nuclear flasks (usually to/from Sellafield, Cumbria). For security and safety reasons, trains carrying nuclear waste cannot be allowed to be left standing after a breakdown. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Double-heading」の詳細全文を読む スポンサード リンク
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