The steam goes through the pipe from the boiler into the piston where it pushes. The piston is connected to an incompressible rod that is attached the wheel, which means that the horizontal motion of the piston pushed on the wheel. The heavy engine pushes down on the wheel so much that there is enough friction for the wheel not to slip. The push of the piston can thus turn the wheel. The drive wheel is coupled to the front wheel. I guess that this means the friction at the front can be utilised as well.
