Actually, a steam locomotive is very well suited to study the crank mechanism of a piston-stroke engine. Everything is visible from the outside. Thus the wheels form the cranks onto which connecting rods deliver their torque in rotation direction. These in turn are driven by pistons which are arranged in front in big black cylinders between the first and the second wheel set. Importantly for these is the control mechanism immediate above. They cause the somewhat confusing lever arrangements. Nevertheless, they must bring the sliding-sleeves in the steering box in the position so that the piston is at any time supplied with enough vapour, from the correct side. So that the locomotive can also drive off in TDC position, the other side of the rigid wheel axles is moved around 90°.
Thus only the (substantially bigger) rest of the machine remains. This is formed almost exclusively by the heating, the pipes for vapour production, and the water tank. By the way, you can also recognise by the outside horizontal crank mechanism whether we are dealing with an express or goods train locomotive. The picture displays an express locomotive, because it has few, but relatively large wheels. The goods train locomotive has more and smaller wheels for more torque and traction. 09/08