Centre-flue Boilers
The simplest boiler for locomotives had a single straight flue. It was widely used by many of the early locomotive makers, including Blenkinsop's locomotives for the Middleton railway and Stephenson's Locomotion.
This type of boiler is simple to manufacture and strong enough to support "high pressure" (for the period) steam with expansive working in the cylinders. There is also good gas flow through the large flue, so that the fire receives sufficient draught from the action of a tall chimney alone. However it also has little heating area, so is inefficient and burns a large amount of coal.
Read more about this topic: Flued Boiler