based on the double acting steam engine with sun-and-planet gear invented by James Watt.
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High pressure steam (red balls) enters from the boiler at the top through a valve arrangement into the cylinder and pushes the piston forward. A secondary rod controls the valve opening to allow steam to enter on the left when the piston is on the left and on the right when the piston is on the right.
At the end of the piston stroke the valve shifts, allowing the expended steam to escape through the exhaust port (underneath the blue valve slide).
Keveney, M. (2000) The Locomotive. http://www.keveney.com/Locomotive.html (accessed 9 March 2004)