"If the notice says 'danger,' you shouldn't pass it."
— Sixteen's friends, Cab Over Wheels, Wilbert the Forest Engine
When Sixteen worked at the steelworks, he worked with other industrial engines who were described by Wilbert as his friends. Unlike Sixteen, they obeyed the rules and never went beyond the "DANGER" notice at the tip. They tried their best to warn Sixteen not to go beyond the "DANGER" notice, to no avail.
Earnest has no specific basis, and appears to be a freelance design. However, he does strongly resemble a Great Western Railway Star Class, the 2-2-2 broad gauge steam locomotive. A non-working replica of a GWR Star Class named North Star was constructed for the 1923 Cavalcade, and is now housed at Swindon Steam Railway Museum.
According to a SiF interview, the portrait of Earnest was illustrated by Robert Gauld-Galliers, and was described to be a "made-up character" in the Victorian era to match the various other human portraits featured throughout the series. The portrait was roughly 80 x 60 mm.
In 2017, Robert Gauld-Galliers confirmed that at the time of production Earnest was not named, and suggested he be named 'Earnest'.
Earnest is the first engine in the television series to have a moustache, and the second overall.