What do you mean by Pendulum principle of transport?

Transportation costs are divided between two producing regions when mutually necessary for the supply of goods. That rule of transport is known as the pendulum principle. The significance of this policy is immense in the expansion of the iron and steel industry in the Lakes region of America. Rail wagons and ships never travel empty, as they travel from the Mesabi and adjoining Panch Lake areas to the Appalachian region with iron ore and to the lake area with coal from Appalachian. It is divided into the coal-producing industries of the Bay Appalachian and the locust-producing industries of the region. The iron and steel industry in Pittsburgh, Youngstone etc. benefited economically as a result of this pendulum policy.