Unlike modern machines, the steam locomotive - which underwent few fundamental changes in 125 years of development - openly displays many of its parts. This mechanical honesty has long captivated onlookers and invited study, but many still wonder just how it all works. In fact, entire books have been devoted to the subject. 

There are two basic areas of activity on a steam locomotive: the boiler where steam is made, and the engine (cylinders, rods, and driving wheels) where steam is used. 

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