Clan Line is driven by high-pressure steam driving a piston backwards and forwards within a cylinder. This motion is transferred to the driving wheels by way of a connecting rod. There are no gears, as there are in a car - the driven axle and associated wheels are the crank shaft. The exhaust from the cylinder goes out of the chimney, and can be heard as a "chuff". The piston is double-acting - that is, steam pushes it one way, and then pushes it the other way. This means that there are two "chuffs" per cylinder for each revolution of the driving wheels. Clan Line has three cylinders, so there are six "chuffs" for each revolution of the driving wheels.

Steam is admitted to the cylinders, and allowed to exhaust up the chimney, by the valves. The movement of the valves can be adjusted by the driver, using a device that is usually known as the reverser. By adjusting the reverser, the driver can change the direction of movement of the locomotive. He can also use the reverser in the same way as the gears in a car - he sets it to one position for starting the train and adjusts it as speed increases, and the power requirement decreases.

The water to produce the steam is heated in the boiler by burning coal. The fire sits on a grate in the firebox. The sides, back and top of the firebox are made of two sheets of steel, with water in between. The boiler barrel is in front of the firebox, and has lots of tubes running through it to the smokebox. These tubes are also surrounded by the boiler water, and the hot exhaust from the fire goes through them. This gives a large surface area which allows the heat from the fire to transfer to the water.

Air gets to the fire from below the grate (primary air), and through the firehole door (secondary air). The fireman can control the amount of primary air by opening and adjusting doors, known as dampers, in the ashpan beneath the grate. He can also adjust the amount of secondary air by adjusting the opening of the firehole door.

As mentioned earlier, the exhaust from the cylinders goes out of the chimney, controlled by the valves. It gets to the chimney via the blast pipe, which is in the smokebox. The blast pipe, and associated equipment, is specifically designed, using aerodynamics, to decrease the air pressure inside the smokebox. This causes combustion products to be drawn through the tubes, and air through the fire. This means that, when the engine is working harder, more air is drawn through the fire, making it burn brighter and generate more heat.

Clan Line in the snow