A locomotive usually, though not always, has a pair of simple engines. These act as ordinary horizontal steam engines, steam being admitted and cut off according to the notching-up. It then expands to fill the cylinder, pushing the piston before it. Just before the end of the stroke the exhaust port opens and steam is exhausted from one side of the piston up the chimney, its pressure, which now is a back press' re or resistance, falling and the piston being pushed by fresh steam in the opposite direction. The motion of the piston is transmitted through the piston and connecting rods to the crank, and thence to the wheels.