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Big Boy is the popular name of the American Locomotive Company 4000-class 4-8-8-4 articulated, coal-fired, steam locomotives manufactured between 1941 and 1944 and operated by the Union Pacific Railroad until 1959. The Big Boy fleet of twenty five locomotives were used primarily in the Wyoming Division to haul freight over the Wasatch mountains between Green River, Wyoming and Ogden, Utah. They were the only locomotives to use a 4-8-8-4 wheel arrangement consisting of a four-wheel leading truck for stability entering curves, two sets of eight driving wheels and a four-wheel trailing truck to support the large firebox. According to Union Pacific senior manager of Heritage Operations Ed Dickens Jr., the 4-8-8-4 series originally was to have been called "Wasatch". One day while one of the engines was being built an unknown worker scrawled "Big Boy" in chalk on its front. With that, the legendary name was born and has stuck ever since.〔 ==Design== Union Pacific introduced the Challenger-type (4-6-6-4) locomotives in 1936 on its main line over the Wasatch Range between Green River, Wyoming and Ogden, Utah. For most of the route, the maximum grade is 0.82% in either direction, but the climb eastward from Ogden, Utah into the Wasatch Range reached 1.14%. Hauling a freight train demanded doubleheading and helper operations, and adding and removing helper engines slowed operations. To eliminate the need for doubleheading and helper operations, Union Pacific decided to design a new locomotive. For such a locomotive to be worthwhile, it would have to be faster and more powerful than slower locomotives like earlier compound 2-8-8-0s that UP tried after World War I. To avoid locomotive changes, the new class would need to pull long trains at a sustained speed of once past mountain grades. In fact, it was designed so that it could travel smoothly and safely at even though it was not intended to be used that fast.〔 Led by mechanic Otto Jabelmann, the Union Pacific Railroad's design team worked with the American Locomotive Company to re-examine their Challenger locomotives. The team found that Union Pacific's goals could be achieved by enlarging its firebox to approximately (about ), lengthening the boiler, adding four driving wheels and reducing the size of the driving wheels from on a new engine. The Big Boys are articulated, like the Mallet locomotive design. They were built with a wide margin of reliability and safety, and normally operated well below in freight service. Peak horsepower was reached at about ; optimal tractive effort, at about . Without the tender, the Big Boy has the longest engine body of any reciprocating steam locomotive. 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「Union Pacific Big Boy」の詳細全文を読む スポンサード リンク
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