A cast-steel piston, attached to guide rods at each end, is fitted tightly into a smoothly-bored cylinder on each side of the locomotive. Above the cylinder is the piston valve, and the rear piston rod is attached to the crosshead when it protrudes from the cylinder. The entire assembly cants sideways to the locomotive at either the top or bottom, depending on the size of the smokebox above it. The piston travels easily inside the cylinder, pulling or pushing the crosshead (and therefore the main rod) as the steam expands against it.
Quick facts
Energy is everything
First use in England
First use in United States