Clockwork engine more

Leading Wheels. These aren't powered and sit in front of the driving wheels. 

Driving Wheels. Driving wheels move a steam locomotive through pistons and rods. Photo

Trailing wheels - trail

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Piston valves are one form of valve used to control the flow of steam within a steam engine or locomotive. They control the admission of steam into the cylinders and its subsequent exhausting, enabling a locomotive to move under its own power.  

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Steam passes to the piston valves which control admission

 of steam to the cylinders. Once in the piston, the steam

 expands, pushing it in the opposite direction. At the end of

 the piston stroke, an exhaust port opens, allowing the steam

 to escape. 

Then the process is reversed and repeated in the opposite direction.

source

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In its most basic form, a bicycle pump functions via

 a hand-operated piston. During up-stroke, this

 piston draws air through a one-way valve into the pump from outside. 

During down-stroke, the piston then displaces air from the pump into the bicycle tyre.

 source

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