The J1234 locomotive passes along the Esk Valley near Napier.

How it works

In the steam locomotive, the boiler lies between the firebox and the smokebox. Coal or oil is used to heat the firebox. Heat from the firebox passes through boiler tubes running along the outside of the boiler. This heats the water in the boiler, and then passes to the smokebox and out through the stack. Steam from the boiler passes through control valves into the main cylinders where it acts on the driving pistons. Each piston is connected to the driving wheels, which are connected to one another by linkages called side rods. In this manner, the linear movement of the pistons is converted into the rotational movement of the wheels.