This type of locomotive was developed by the Union Pacific on the basic model articulated Mallet. They were built between 1936 and 1943. They correspond to the model Mallet by being articulated, but all cylinders are directly powered by live steam at high pressure while in the model Mallet both are trained by the steam at high pressure and two steam recycle more low pressure. This gave more power to the hook with more consumption of fuel with a lower efficiency that was not the main concern. The articulated model can take tighter curves. The Union Pacific was 105 Challengers and other companies had also. We solved this model of behavior problems in high-speed line for the geared locomotives and then allowed it to build even larger locomotives and fast like the 4-8-8-4 Big Boy.

There were two models which had the second largest centipede tender.

One,  No 3985,  , is kept in working order by the Union Pacific in Cheyenne Wyoming, and is the largest locomotive in running the world. She served excursion tours until 1990.
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