A working locomotive has connecting rods which are pulled and pushed by the pistons, and coupling rods which make all the driving wheels turn together. When the engine is working, these rods together with the crank produce forces which are to some extent balanced by weights cast as part of the wheels.
Above I show three different designs. The old engine at Kohukohu has the distinctive pattern at the top, while nearly every other engine has one of the two lower patterns. The Kohukohu engine also has 7-spoked wheels, which seems to be unusual.