Mr. Fergus Duncan, nicknamed the Small Controller, is the controller of the Arlesdale Railway. He is taller than the Fat Controller and the Thin Controller; his nickname refers to the fact that his railway is often called the "Small Railway".
He was appointed by the Fat Controller, the Thin Controller, and the Owner to run the line so they could get the supplies of ballast that reside in the hills on the line.
The Small Controller is very passionate about his work, and never married. He preferred instead to devote his life to work on his railway.