I ran across this looking for something else. In logging, a steam engine mounted on a suitable carriage was used to pull logs with a cable for several hundred yards. The logs were then loaded onto rail cars and hauled to the mill. The rails were referred to as a dummy line, and rural county roads were often built in the roadbed after the rails and ties were moved.

The cable engine was known as a "donkey engine" and sometimes as a "dummy engine," probably because of the similarity of the words donkey and dummy.